Intraoperative neurophysiological monitoring system

ABSTRACT

An intraoperative neurophysiological monitoring system includes an adaptive threshold detection circuit adapted for use in monitoring with a plurality of electrodes placed in muscles which are enervated by a selected nerve and muscles not enervated by the nerve. Nerve monitoring controller algorithms permit the rapid and reliable discrimination between non-repetitive electromyographic (EMG) events repetitive EMG events, thus allowing the surgeon to evaluate whether nerve fatigue is rendering the monitoring results less reliable and whether anesthesia is wearing off. The intraoperative monitoring system is designed as a “surgeon&#39;s monitor,” and does not require a neurophysiologist or technician to be in attendance during surgery. The advanced features of the intraoperative monitoring system will greatly assist neurophysiological research toward the general advancement of the field intraoperative EMG monitoring through post-surgical analysis. The intraoperative monitoring system is preferably modular, in order to allow for differential system pricing and upgrading as well as to allow for advances in computer technology; modularity can also aid in execution of the design.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to surgical apparatus and moreparticularly to a neurophysiological monitoring system including a nerveintegrity monitoring instrument for use in conjunction with one or moreelectrical stimulus probes as an intraoperative aid in defining thecourse of neural structures. The invention is particularly applicablefor use in monitoring facial electromyographic (EMG) activity duringsurgeries in which a facial motor nerve is at risk due to unintentionalmanipulation, although it will be appreciated that the invention hasbroader applications and can be used in other neural monitoringprocedures.

[0003] 2. Discussion of the Prior Art

[0004] Despite advances in diagnosis, microsurgical techniques, andneurotological techniques enabling more positive anatomicalidentification of facial nerves, loss of facial nerve function followinghead and neck surgery such as acoustic neuroma resection is asignificant risk. Nerves are very delicate and even the best and mostexperienced surgeons, using the most sophisticated equipment known,encounter a considerable hazard that a nerve will be bruised, stretchedor severed during an operation. Studies have shown that preservation ofthe facial nerve during acoustic neuroma resection may be enhanced bythe use of intraoperative electrical stimulation to assist in locatingnerves. Very broadly stated, the locating procedure, also known as nerveintegrity monitoring, involves inserting sensing or recording electrodesdirectly within cranial muscles enervated or controlled by the nerve ofinterest. A suitable monitoring electrode is disclosed in U.S. Pat. No.5,161,533 (to Richard L. Prass et al.), the entire disclosure of whichis incorporated herein by reference.

[0005] One method of nerve localization involves the application ofelectrical stimulation near the area where the subject nerve is believedto be located. If the stimulation probe contacts or is reasonably nearthe nerve, the stimulation signal applied to the nerve is transmittedthrough the nerve to excite the related muscle. Excitement of the musclecauses an electrical impulse to be generated within the muscle; theimpulse is transferred to the recording electrodes, thereby providing anindication to the surgeon as to the location of the nerve. Stimulationis accomplished using hand held monopolar or bipolar probes such as theElectrical Stimulus Probe disclosed in U.S. Pat. No. 4,892,105 (toRichard L. Prass), the entire disclosure of which is incorporated hereinby reference. The probe of Pat. No. 4,892,105 has become known as thePrass Flush-Tip Monopolar Probe and is insulated up to the distal tip tominimize current shunting through undesired paths. An improved structurefor a bipolar probe is disclosed in the provisional patent applicationentitled Bipolar Electrical Stimulus Probe (filed Aug. 12, 1998,application number 60/096,243), the entire disclosure of which is alsoincorporated herein by reference.

[0006] Another method of nerve localization involves mechanicalstimulation of the nerve of interest by various dissecting instruments.Direct physical manipulation of a motor nerve may cause the nerve toconduct a nerve impulse to its associated musculature. If those musclesare being monitored using a nerve integrity monitoring instrument, thesurgeon will hear an acoustic representation of the muscle response inclose temporal relationship to the antecedent mechanical stimulation.This will allow the nerve of interest to be roughly localized at thecontact surface of the dissecting instrument.

[0007] Prior art nerve integrity monitoring instruments (such as theXomed® NIM-2® XL Nerve Integrity Monitor, manufactured by the assigneeof the present invention) have proven to be effective for performing thebasic functions associated with nerve integrity monitoring such asrecording EMG activity from muscles innervated by an affected nerve andalerting a surgeon when the affected nerve is activated by applicationof a stimulus signal, but have significant limitations for some surgicalapplications and in some operating room environments.

[0008] A first problem is users have noticed certain EMG measurementartifacts have a disruptive effect on monitoring and tend to causeundesirable false alarms. In particular, EMG monitoring often isperformed during electrocautery in a surgical procedure, whereinpowerful currents surge through and cauterize the tissue, often todevastating effect on the monitor's sensitive amplifier circuits.Electrocautery can also induce an undesired direct current (DC) offsetfrom buildup of charge on the monitoring or sensing electrodes or withinrecording amplifier circuitry. A method of muting during periods ofelectrocautery using in-line detection of electrocautery, based uponfrequency and amplitude was disclosed in Prass, et al.: “Acoustic(Loudspeaker) Facial Electromyographic Monitoring: EvokedElectromyographic Activity”, Neurosurgery 19: 392-400, 1986; and animproved method involving an inductive probe pickup was described inU.S. Pat. No. 4,934,377, entitled “IntraoperativeNeuroelectro-physiological Monitoring System”, by Prass, et al., theentire disclosures of which are incorporated herein by reference.

[0009] Brief pop noise in the form of high frequency bursts (caused byspurious electromagnetic and current artifacts or when non-insulatedmetal instruments are accidentally brought into physical contact) may berecorded during nerve integrity monitoring. These brief artifacts may beconfused for true electromyographic (muscle) responses and may lead tomisinterpretation and false alarms, thereby reducing user confidence andsatisfaction in nerve integrity monitoring. Maintenance of highcommon-mode rejection characteristics in the signal conditioning pathhas helped to reduce such interference, however, false alarms stilloccur. Any solution tending to eliminate or minimize false alarmproblems would increase the accuracy and effectiveness of monitoringprocedures.

[0010] Prior art nerve integrity monitoring devices incorporate a simplethreshold detection method to identify significant electrical eventsbased upon the amplitude of the signal voltage observed in themonitoring electrodes, relative to a baseline of electrical silence, amethodology having disadvantages for intraoperative nerve integritymonitoring. Use of intramuscular electrodes in close bipolar arrangement(as described in U.S. Pat. No. 5,161,533, cited above) provides adequatespatial selectivity and maintenance of high common mode rejectioncharacteristics in the signal conditioning pathway for reducedinterference by electromagnetic artifacts, but yield a compresseddynamic range of electrical voltage observed between the pairedelectrodes. When physically situated near one of the electrodes, asingle nerve motor unit (e.g., activation of a single nerve fiber) maycause an adequate voltage deflection to be heard (by a surgeon listeningto the EMG audio signal feedback) as a clear signal spike or exceeding apredetermined voltage threshold. Moreover, with close electrode spacingand bipolar amplification, recording of larger responses is frequentlyassociated with internal signal cancellation, significantly reducing theamplitude of the observed electrical signal. The resultant compresseddynamic range is advantageous for supplying direct or raw EMG audiosignal feedback to the operating surgeon, in that both large and smallsignal events may be clearly and comfortably heard at one volumesetting, but an EMG audio signal feedback having compressed dynamicrange provides limited ability to fractionate responses based uponmagnitude of the response or obtain an accurate measure of signal power.Another disadvantage of prior art methodology of threshold detection isthat the surgeon cannot readily distinguish or select between electricalartifacts and EMG activity.

[0011] A second problem is that the nerves of interest may frequentlyexhibit a variable amount of irritability during the surgical procedure,which may be caused by a disease process or by surgical manipulationssuch as mild traction or by drying or thermal effects. Such nerveirritability is recorded by nerve integrity monitoring electrodes and isdisplayed and annunciated to the operating surgeon as a series of“beeps” caused by repetitive triggering of threshold detection or byrepetitive electromyographic spikes. Because nerve irritability does notappear in close temporal relationship to particular surgicalmanipulations, it provides no localizing information. When suchrepetitive activity is observed, the surgeon usually ceases all ongoingsurgical manipulations and may irrigate the surgical field in an attemptto reduce nerve irritability. Once a reasonable effort to reduce nerveirritability has been carried out, any residual nerve irritabilitybecomes “noise” and may interfere with the ability to detectelectrically and mechanically stimulated nerve activity. Any methods toreduce the effect of background nerve irritability on detection of briefbursts of nerve activity would enhance localization of nerves ofinterest during periods of increased nerve irritability.

[0012] A third problem arises when monopolar probes, bipolar probes orelectrified instruments are selected for electrical stimulation duringintraoperative neurophysiological monitoring. Each type of probe has itsown advantages, disadvantages and “best application” duringintraoperative procedures. Because of a variable tendency for currentshunting, the optimum stimulus intensity may vary significantly amongprobes. For a given probe type, the ideal stimulus intensity is lowenough to allow spatial selectivity, but high enough to avoidfalse-negative stimulation as a result of current-shunting or otherinfluences. The commercial EMG-type nerve monitors of the prior art havea single current-source terminating in either one or two outputs. Ifthere are two outputs, the outputs are connected in parallel with asingle common stimulus intensity setting and so there is no ability toprovide separate (optimized) stimulus intensities or to guard againstparallel communication between the two outputs. If both outputs areconnected to stimulus instruments, undetected current-leak could occurthrough parallel channels and result in false-negative stimulation. Atleast one manufacturer or prior art monitoring instruments offers aswitchable connector at the stimulus probe terminus, allowing more thanone stimulus instrument to be kept in readiness, and avoiding parallelconnections to the unused instruments, but performing the act ofswitching requires a surgical staff member such as a nurse or technicianand so is cumbersome and, being time consuming, expensive.

[0013] A related problem is that prolonged nerve irritability may be dueto light anesthesia, rather than to inherent nerve irritability. Anymethod to distinguish these two possibilities would enhanceinterpretation during nerve integrity monitoring.

[0014] Another problem confronting users of prior art nerve integritymonitoring devices is that quantative measurements of nerve function arerelatively cumbersome to obtain, since equipment setting changes must beperformed by operating room personnel while electrical stimulationprocedures are performed by the operating surgeon. For example, athreshold determination for electrical nerve stimulation is an acceptedindication of functional nerve integrity. Determination of responsethreshold requires stimulation at multiple stimulus intensities, whichmust be changed manually, and nerve responses must be recorded at eachstimulus intensity level. With prior art technology, this process istime-intensive and discourages serial determinations during theoperation as an ongoing measure of nerve integrity. Thresholddeterminations are typically performed only at the end of the operativeprocedure as a prediction of immediate postoperative function. Whenusing prior art methods, if the threshold is found to be abnormal, thesurgeon is usually unaware of when the change to abnormality occurredduring the operative procedure. Any method making quantitativemeasurements of nerve function convenient and rapid to obtain wouldenhance nerve integrity monitoring.

[0015] Another concern is how functions are controlled. There is arelatively strong conceptual separation between off-line control(performed at some time other than during the procedure) and on-linecontrol (performed during a surgical procedure), as pertains to controlof intraoperative neurophysiological monitoring system functions throughthe use of input devices. “Off-line” operations are performed whenmonitoring is not actively being performed, for example, as whenlogging-in patient information, setting system preferences or retrievingsaved-data for “post-production” analysis, whereas “on-line” refers toperiods of active intraoperative neurophysiological monitoring.

[0016] In prior art nerve integrity monitoring devices, controls foroff-line functions consist of front panel knobs and switches or keyboardand mouse with proprietary software to perform common setup functionsand parameter adjustments. Additional back panel switches may beavailable to adjust less commonly changed parameters, such as stimulusrate and duration. For multi-channel nerve integrity monitoring withqualitative and quantitative signal analysis, front and back panelhardware is cumbersome and too limited in scope. Greater flexibility andconvenience in off-line controls is available through use keyboard andmouse input and software capabilities to modify and store setupinformation in archival files for facilitation of off-line setupfunctions. A limitation of prior art strategies is that the setupinformation is held in volatile memory during actual monitoringoperations, rendering the setup information vulnerable to strongelectrical surges, electromagnetic noise or accidental powerinterruptions. An electrical surge or accidental unplugging may causeloss of all new (different from “default”) setup information, requiringa “reboot” of the system and adjustment to get back to the desiredsettings. Any method for off-line control allowing similar flexibility ato keyboard and mouse input and having the convenience of designatedsoftware with archival (file) storage of setup information, but withoutrisk of erasure by spurious electrical events or accidental equipmentunplugging, would represent a significant advance for nerve integritymonitoring. Stimulation devices of prior art for neurophysiologicalmonitoring are manually controlled through front panel potentiometersand switches or with mouse and keyboard to produce paired or burststimuli and stimuli of opposite polarity in an alternating pattern, butlack the ability to deliver consecutive stimuli of differing intensitiesor alter the pattern of stimulation at a predetermined time without thattime consuming manual input. Analogously, none of the monitoringinstruments of the prior art provide delivery of selected stimuli incoordination with data acquisition, analysis, display, and storage.Moreover, In prior art nerve integrity devices, control of on-linefunctions is performed by keyboard and mouse or by front panel controlsand, because of a possible breach of sterility, the operating surgeoncannot perform such functions by himself or herself and so changingequipment settings requires involvement of hospital personnel at therequest of the operating surgeon and may be time-consuming, cumbersomeand possibly risky, since the changed settings may be inaccurate. Anymethod allowing rapid and accurate changes in equipment function withoutthe need of ancillary operating room personnel and without risk tomaintenance of sterility would be considered an enhancement of nerveintegrity monitoring.

[0017] An important function of intraoperative neurophysiologicalmonitoring is detecting brief episodes of EMG activity, caused by directelectrical and mechanical stimulation. Detection allows the surgeon tolocalize a nerve of interest approximately at the contact surface of thedissecting or stimulating instrument. Detection of brief, localizing EMGactivity is frequently obscured by the presence of repetitive EMGactivity caused by “baseline” nerve irritability. Such irritability maybe due to nerve compromise caused by the disease process itself or tovarious surgical manipulations, such as mild traction, drying, thermalstimulation, or chemical irritation. When significant repetitiveactivity is observed, the surgeon typically ceases all surgicalmanipulations and may irrigate the wound in an attempt to “quiet” nerveirritability. Once a reasonable attempt has been made to allow the nerveto become quieted, any remaining repetitive activity is essentially“noise” and may interfere with hearing more important brief EMGresponses that allow localization of the nerve of interest. Suchbackground irritability is particularly a problem during acousticneuroma resections, which is one of the most common procedures for whichfacial nerve monitoring is used.

[0018] Redundancy afforded by multi-channel monitoring of (single)nerves of interest provides some opportunity to maximize the ability todetect localizing information during periods of problematic repetitive(non-localizing) activity. The most common application of nerveintegrity monitoring involves monitoring the facial nerve. The facialnerve has a long course, beginning in the cranial cavity, then through abony channel (fallopian canal) within the temporal (ear) bone, exitingbehind the ear to swing forward and innervate the nerves of the facialexpression. The nerve is at risk during a number of surgical proceduresinvolving the ear, the temporal bone and intracranially. Intracranially,and in its course through the temporal bone, the nerve appears as asingle nerve bundle, with no internal topographical organization. As thenerve exits the temporal bone behind the ear it finally separates intotwo major trunks, which further divide into 5 major branches.Multi-channel nerve integrity monitoring of the facial nerve involvesplacing electrodes into multiple facial muscles, representing multiplebranches of the nerve. While not necessarily the preferred approach, thelack of topographical organization of the intracranial and intratemporalportions of the facial nerve, allows monitoring during removal ofacoustic neuromas and during ear surgery with only one or twoelectromyographic channels.

[0019] Multichannel monitoring of the facial nerve is preferred in orderto increase sensitivity and to provide redundancy in the event ofelectrode failure. Redundant facial nerve monitoring channels alsoprovides flexibility to maximize the ability to detect localizing, briefnon-repetitive EMG activity. The upper and lower facial musculature havebeen observed to have differential tendencies to exhibit mechanicallyevoked EMG activity. The lower face tends to be more sensitive ineliciting mechanically-stimulated EMG activity but also has a greatertendency to exhibit “background” nerve irritability. During periods whenbackground repetitive EMG activity obscures auditory detection of moreimportant and localizing non-repetitive activity, the most active EMGchannels can be deleted (muted) from the signal directed to the surgeonthrough audio loudspeaker(s). The remaining EMG channels, having lesscompeting background noise to interfere, are more easily heard by theoperating surgeon in order to detect (localizing) mechanically andelectrically stimulated EMG activity.

[0020] The majority of prior art nerve integrity monitoring devices haveonly two channels, which allows little redundancy and flexibility. Whenrepetitive activity becomes bothersome and persistent, despitereasonable efforts on the part of the operating surgeon to allow thenerve to quiet down, the surgeon may ask an operating room employee to“turn the monitor down.” This solution is problematic, because it maycause the surgeon to miss hearing important localizing EMG information.Alternatively, with the availability of multiple (redundant) EMGchannels, a nurse or operating room technician may individuallyeliminate each electrode channel in an attempt to identify the offendingchannels, so that they may be (temporarily) eliminated. This process maybe greatly facilitated, if there is some visual indication of relativeEMG activity among the various EMG channels. However, even with visualdisplays, the process may still be time consuming and, therefore,expensive. Moreover, once certain “offending” channels have been muted,there may be long periods before the surgeon, the nurse, or operatingroom technician remember or “feel safe” to add these channels back tothe audio signal. This may cause unnecessarily long periods of decreasedsensitivity.

[0021] There is a need, then, for a nerve integrity monitoringinstrument having greater flexibility and stability in use, greatersensitivity and specificity (e.g., noise rejection and artifactidentification), and a user interface more readily adapted to performingthe monitoring procedures required without distraction to the surgeonwhile concentrating on the medical aspects of surgical procedure.

OBJECTS AND SUMMARY OF THE INVENTION

[0022] Accordingly, it is a primary object of the present invention toovercome the above mentioned difficulties by providing an improvedmethod and apparatus for sensing and/or recording of electrical activityin the nerve tissue.

[0023] Another object of the present invention is enabling a surgeon toelectrically stimulate, record, analyze and store (or archive)electrical activity in nerve tissue without requiring concurrentperformance of distracting instrument adjustment procedures.

[0024] Another object of the present invention is to provide amultichannel nerve integrity monitor having improved resistance to thedeleterious effects of spurious signal artifacts.

[0025] The aforesaid objects are achieved individually and incombination, and it is not intended that the present invention beconstrued as requiring two or more of the objects to be combined unlessexpressly required by the claims attached hereto.

[0026] In accordance with the present invention, an intraoperativeneurophysiological monitoring system includes a number of novelfeatures, including: a digitally controlled stimulator having multipleindependant stimulus outputs; an artifact detection electrode withmodified wire leads to enhance its sensitivity for recording electricalartifacts; a novel method and algorithm for detecting brief artifactsusing the artifact detection electrode and an enhanced method andalgorithm for threshold detection; a method and algorithm forcontrolling the sequence of monitoring events controlled by detection ofprobe contact with tissue; and a method and algorithm for controllingoperation of the nerve interity monitoring system in which theelectrical stimulus probe is used as a computer pointing or inputdevice.

[0027] The intraoperative neurophysiological monitoring systemstimulator preferably includes a nerve integrity monitoring instrumenthaving multiple independent stimulus outputs to provide optimal presetstimulus output parameters for more than one probe type, therebyallowing all probes to be connected at the beginning of the case andused as needed, without delay or confusion related to switching andintensity setting changes. Independent, electrically isolated outputsalso eliminate parallel connections among stimulus probes and possiblecurrent leakage between probes. An optimum number of stimulus outputs ispreferably in the range of two to four. In an exemplary embodiment threestimulus outputs include a monopolar probe, a bipolar probe and anelectrified instrument, all three simultaneously connected.

[0028] For the purposes of nerve integrity monitoring, an electricalstimulus probe is used for locating and defining the contour of thenerve of interest. During such “mapping” procedures, the stimulus probeis moved about the surgical field or along the nerve contour in smallcontrolled steps, during which the stimulus probe is in continuouscontact with tissue, usually for less than one or two seconds.Alternatively, during quantitative measurements of nerve function, thestimulus probe may be applied to the nerve continuously for a few orseveral seconds allowing capture of electromyographic activity foranalysis. Thus, if the stimulus probe is in contact with tissue for lessthan one or two seconds, it may be taken that the surgeon is simplylocating or mapping the contour of the nerve of interest. If continuoustissue contact exceeds one or two seconds, the surgeon's intent islikely to be otherwise, such as for quantitative measurements. Further,if the stimulus probe is tapped twice or three times onto patienttissue, the temporal pattern of continuous tissue contact is quitedifferent from either of the previous patterns and might be consideredas a “request” by the surgeon.

[0029] The present invention incorporates a method of controlling avariety of nerve integrity monitoring functions through detection of theduration of continuous contact of a designated stimulus probe withpatient tissue. Alternative methods to more accurately detect thetemporal pattern of continuous contact of the stimulus probe withpatient tissue include continuous measurement of stimulation circuitimpedance and measurement of current flow using a continuous, distinct(second) subthreshold current, delivered “downstream” from the actualelectrical stimulus. Continuous measurement of stimulation circuitimpedance is the preferred method and provides the following benefits:

[0030] 1. A quality check of the stimulus probe and circuit. Flush tipprobes (e.g., as described in U.S. Pat. No. 4,892,105 and provisionalpatent application No. 60/096,243, filed Aug. 12, 1998, the entiredisclosures of which are incorporated herein by reference) have acharacteristic impedance, based partly on the cross-sectional area ofthe conductor. A measured characteristic impedance that is significantlybelow the expected characteristic impedance is sensed and indicates aparallel current path or a breach of insulation.

[0031] 2. Indication of tissue contact with stimulus probe. Detection oftissue contact is used to drive a preset sequence of events, asdiscussed in greater detail, below.

[0032] 3. A definitive solution for minimizing stimulus-pulse-relatedrecording artifacts on other equipment by permitting current to flowonly during tissue contact with a stimulus probe. Detection of acharacteristic impedance decrease in the stimulus circuit during tissuecontact with the stimulus probe is sensed and, in response, a relayswitch is activated, allowing current flow to the appropriate probe. Ifa single current source is used to drive multiple stimulus outputs,detection of a characteristic impedance decrease at a stimulus probeoutput triggers driving relay switches to “open circuit” other stimulusprobe circuits in order to definitively eliminate parallel connectionswith other outputs.

[0033] 4. While controversial, constant voltage has been cited as moreadvantageous than constant current for purposes of electricalstimulation, as a means to reduce the occurrence of false-negativestimulation in the setting of stimulus shunting (Moller A, Janotta J.:Preservation of facial function during removal of acoustic neuromas: useof monopolar constant voltage stimulation and EMG. J. Neurosurg 61:757-60, 1984). Since stimulus current is the aspect relating to stimulusadequacy and injury potential, most applications incorporate constantcurrent stimulus sources for greater accuracy and safety in stimulusdelivery. Continuous measurement of stimulus circuit impedance allows a“best of both worlds” opportunity. Stimulus probes and electrifiedinstruments have characteristic or optimal impedance values based uponthe contact surface of the particular instrument. A reduction ofimpedance below that of the characteristic value is taken as anindication of stimulus shunting, presumably away from the area intendedfor electrical stimulation. This is particularly apt to occur with useof electrified instruments, where the Insulation is not carried all theway to the tip so as to not Interfere with Its surgical use. Incombination with digital control of stimulus parameters, detection of astimulus circuit impedance decrease below the pre-determined “optimal”value is used to trigger a compensatory increase in delivered stimuluscurrent in a pre-determined fashion. The rate of change or slope ofcurrent increase, relative to the amount or percentage of impedancedecrease, is preselected for aggressive or less aggressive compensationpatterns and an upper limit of current increase is also predeterminedfor safety considerations. Such a compensatory current increase saferand reliable than simple use of constant voltage.

[0034] 5. The impedance detection circuit provides a mechanism enablinguse of the stimulator probe as an input device.

[0035] Additional circuitry is required for impedance detection, with anadditional patient connection electrode having its own isolation, and anadditional continuous, subthreshold probe signal (i.e., below thethreshold required for nerve activation) must be delivered through theprobe tip for measurement by the impedance detection circuit.

[0036] In an alternative embodiment, a continuous, second subthresholdcurrent is delivered to the stimulus probe, downstream from the pulsedcurrent used for actual nerve stimulation. Detection of flow of thecontinuous current provides more accurate detection of tissue contactthan for pulsed stimulation alone and permits detecting a “tapping”pattern of the stimulus probe. Continuous current flow detection doesnot provide as many possible benefits as continuous stimulus circuitimpedance measurement, but also does not require placement of anadditional patient electrode and the necessary isolation circuitry.

[0037] In addition to detecting and responding to a temporal pattern ofcontinuous tissue contact of the stimulus probe, the present stimulatoris adapted for digital control. Stimulus intensity, pulse duration, andtemporal pattern of stimuli presentation are controlled through adigital controller having an interface circuit. The interface storespre-programmed stimulus algorithms or paradigms, preferably in nonvolatile memory. The stimulus paradigms are preferably constructedoff-line using appropriate stimulus control algorithm developmentsoftware and is preferably loaded or burned into a non-volatile ReadOnly Memory (ROM) chip, included within the interface. During amonitoring procedure, contact with tissue will trigger a predefinedsequence of events called, for purposes of nomenclature, a TissueContact Initiated (TCI)-Timeline, thereby activating the stored stimulusparadigms in a pre-programmed manner.

[0038] Front panel controls consist of basic stimulus intensitycontrols. Stimulus, pulse duration and pulse repetition rate arepreferably adjusted in a limited manner by recessed DIP-switches orother user-accessed, but less prominent controls. The remainingstimulator controls are actuated through a CPU interface, such as via aPCI bus. As discussed above, monitoring parameters and complex stimulusparadigms are stored via non volatile, programmable memory (e.g., flashmemory, EEPROM). The digitally controlled stimulator executing the TCIevent-sequencing time line also communicates with a CPU based datastorage and analysis apparatus to direct binning of responses and totrigger archival data storage, analysis and display paradigms.

[0039] In addition to an indication of which stimulator is active andwhether adequate current delivery is achieved, there is preferably alsoan additional indicator annunciating detection of an adequate targetimpedance, thereby providing a rough quality check of the stimulus probeand the entire stimulator circuit. This type of diagnostic would be bestapplied to the flush tip stimulus probe designs (as in U.S. Pat. No.4,892,105), where the impedance is typically related to thecross-sectional area of the conductor contact surface.

[0040] The controller software used in monitoring the stimulus probeimpedance detection circuit (or current flow detection circuit) includesan algorithm for identifying a pattern of changing impedance (or currentflow change) caused by double or triple taps of the stimulator againstpatient tissue. When double or triple tap patterns are detected, signalsare sent to the circuitry in the CPU digital interface for triggeringpredetermined manipulations. These command signals are preferablyrendered “context sensitive” by their temporal occurrence in relation tothe TCI-Time line.

[0041] Turning now to another aspect of the monitoring system of thepresent invention, a method is provided for detection and identificationof artifacts as an aid to interpretation. For the purposes of thisdescription, “intelligent” refers to electrode sites Involving important“monitored” muscles, supplied or enervated by a particular nerve ofInterest. Non-intelligent refers to other electrode sites within oroutside of muscles, not supplied by the nerve of Interest. Currentartifacts and electromagnetic field noise may best be detected by aspecially constructed electrode that is inserted proximate to therecording field, but not in the (intelligent) muscles supplied by thenerve being monitored. Electrical events, simultaneously recorded inboth “intelligent” electrodes (placed in muscles supplied by the nervebeing monitored) and a “non-intelligent” artifact detection electrode,may be unambiguously interpreted as electrical artifacts. If theartifact detection electrode is placed in a nearby (non-intelligent)muscle not supplied by the nerve being monitored, it may also serve todetect light anesthesia. If repetitive EMG activity is simultaneouslyobserved in monitored muscles and other muscles, it may be interpretedthat the patient is beginning to wake up from anesthesia. Theanesthesiologist may use this information to maintain adequate levels ofanesthesia throughout the procedure. The operating surgeon may also bereassured that the observed nerve irritability is not related tosurgical manipulations. The artifact detection strategy involves theconstruction of an artifact-detection electrode, preferably theelectrode of the present invention is a modification of the electrodedesign of U.S. Pat. No. 5,161,533 (as discussed above). The modificationprovides a greater impedance imbalance between the two electrode leads,thereby reliably enhancing the antenna-like qualities of the probe andthe susceptibility for detecting current and electromagnetic artifactsoccurring in the immediate proximity of multiple standard electrodesplaced in muscles supplied by the nerve of interest.

[0042] The artifact detection electrode of the present invention has anactive-portion that is similar to the paired, bipolar Teflon coatedneedle electrodes, but differs in that the area of un-insulated needleis dimensioned and/or made of a suitable material to provide a reliablydetectable impedance imbalance.

[0043] Preferably, the wire leads are also modified such that the leadlength is approximately 6 inches longer than standard length. The extra6-inch portion is looped over the recording field to create,effectively, an antenna over the recording field. The looped portion istreated to enhance its antenna-like properties. Optionally, incombination with or instead of using differing uninsulated areas ofneedle insertion portion, a resistor is placed in series with one of thetwo electrode leads, thereby creating a readily detected impedanceimbalance, the value of which may be selected (or, with a potentiometer,adjusted) to be within a range of, preferably, zero to approximately50,000 ohms. The resistor is preferably located on the wire lead orloop, or it may be incorporated into an associated electrical connectorhousing or connector body. A relative disadvantage of using a singlestandard recording electrode for detection of electromagnetic field andcurrent artifacts is that the single electrode may not adequatelyrepresent the electromagnetic field for multiple active recordingelectrodes. The loop design, needle to insulation symmetry, fixedresistor value and relative location are the physical factorsdetermining the “antenna like” properties of the electrode design; thevarious features are preferably “tuned” to obtain the optimum electrodecharacteristics. The electrode must be spatially selective enough toavoid pick up of “intelligent” signal, but must have adequate antennalike qualities to provide EM-field and current artifact detection torepresent the entire recording field.

[0044] The uninsulated portion of the electrode needles of the artifactdetection electrode is placed in a proximate, “non-intelligent” muscle,not enervated or supplied by the nerve being monitored. The loopedportion of the electrode lead is placed over the recording field of theintelligent electrodes and held in place, preferably with tape.

[0045] The artifact-detection electrode output is detected and analgorithm incorporating a simple artifact-recognition strategy, basedupon response distribution, is employed. The signal output of theartifact detection electrode is amplified along with that of standard“intelligent” electrodes. Brief supra-threshold signal episodes (approx.<1 sec.), detected in intelligent electrodes, trigger a logic-circuit toevaluate for simultaneous signal in the artifact-detection electrode.Simultaneous detection of supra-threshold signal in theartifact-detection electrode renders an interpretation of “artifact.” Ifno simultaneous signal is detected in the artifact-detection electrode,the episode is interpreted as EMG in the algorithm, since it is highlyunlikely that two different nerves are simultaneously (mechanically orelectrically) stimulated.

[0046] For repetitive EMG activity lasting from several seconds toseveral minutes, detection of activity among “intelligent” electrodesindicates irritability in the nerve of interest, which may be due tosurgical manipulations, whereas simultaneous detection of activity inintelligent and non-intelligent electrodes are interpreted as inadequateor “light” anesthesia, because surgically-evoked repetitive-EMG activityis otherwise unlikely to occur simultaneously in two distinct musclegroups.

[0047] An example of such an artifact detection strategy is the use of amasseter muscle electrode during facial nerve monitoring. The massetermuscle is in the proximate electromagnetic field of the facial muscles,but is not enervated by the facial nerve. Brief electromagnetic andcurrent events that are simultaneously detected in facial and massetermuscles are readily interpreted as artifacts. Further, when repetitiveactivity is detected in masseter and facial electrodes, it suggests thatthe anesthesia is getting light.

[0048] The intraoperative neurophysiological monitoring system of thepresent invention includes a controller circuit and software algorithmsto identify and categorize artifacts based upon the observeddistribution among “intelligent” and “non-intelligent” electrode sites.In one embodiment, a logic circuit receives output from thresholddetection circuits related to both “intelligent” and “non intelligent”electrode sites. When a supra threshold signal is detected in one of the“intelligent” electrode sites, the circuit becomes activated to make adetermination regarding whether the signal detected was likely to havebeen artifact or true EMG. At the time of supra threshold signaldetection in one (or more) of the “intelligent” channels, the output ofthe “non intelligent” channel threshold detection circuit is checked forsimultaneous activation (using, e.g., a logic AND gate). If there was nosupra threshold activity in the “non intelligent” channel, the logiccircuit produces an output signal indicating that the observed activitywas “true EMG”. If simultaneous supra threshold activity was detected inboth the “intelligent” and “non-intelligent” channels, the logic circuitproduces an output signal indicating that the observed activity waslikely to have been a non-EMG artifact.

[0049] The accuracy of the present artifact-detection strategy isdependent upon the strength of the recorded signal. Weak signals thatonly appear in a single channel may not distribute among Intelligent andnon-Intelligent electrodes as predictably as when multiple electrodesare activated.

[0050] If more than one “intelligent” channel (and electrode) isutilized, the logic circuit is preferably configured to allow a userselected requirement to produce an output signal indicating the identityof a supra-threshold signal as “true EMG” or “artifact” only when two ormore “intelligent” channels are simultaneously activated by suprathreshold signals. This will increase the accuracy of the logic circuitdeterminations, reduce the frequency at which the circuit gives falsepositive feedback, and indicate a response of greater magnitude andprobable significance.

[0051] The novel artifact-detection electrode and logical strategy fordistinguishing electrical artifacts and EMG signals of the presentinvention works with simple threshold detection involving analog voltagemeasurement, but simple threshold detection has significant limitationsfor this application. One disadvantage is that repetitive EMG activity,caused by persistent nerve irritability, impairs the ability to detectmore important episodes of non-repetitive EMG activity. Repetitiveactivity swamps the threshold detection circuit and causes repetitivedetection of supra-threshold events.

[0052] In the present embodiment, threshold detection is improvedthrough the use of digital signal processing (DSP), whereby all recordedelectrical activity is digitized and evaluated for mathematicalproperties. A preferred measurement for EMG activity is rectified rootmean square (rRMS), which gives a greater dynamic range for EMG activitymagnitude, as detected by standard electrodes (e.g., as in U.S. Pat. No.5,161,533, discussed above). The greater dynamic range capabilityimproves the ability to distinguish responses, based upon the magnitudeof signal power. For example, while electrical artifacts and EMGresponses show considerable overlap, the peak signal power of anon-repetitive (localizing) EMG activity is usually significantly higherthan for a repetitive (non-localizing) EMG activity. The digitallyprocessed rRMS data stream for each recording channel is continuouslyanalyzed by software for peak and average power within a variable time(probe) window. The width of the probe window (or dwell) over whichpower is analyzed may be varied in width (duration) up to one second,which may be “tuned” to give desired fractionating tendencies. Forexample, If a minimum average power value is used for determining theevent detection threshold, a narrow dwell time will reduce the dynamicrange and improve detection of brief responses. Lengthening the dwelltime will increase the dynamic range and favor selection of only largeroverall responses. Alternatively, use of peak power determinationseffectively neutralizes the effect of response duration, but may havethe greater ability to distinguish repetitive and non-repetitiveresponses. Predetermined criteria for threshold detection may includeminimum values for average power, peak power or both in some combinationor ratio. The use of two distinct probe windows (described inNon-Provisional Patent Application No. ), separated by a variable time(inter-probe Interval) allows greater accuracy in distinguishing briefnon-repetitive (<1.0 sec) and longer repetitive (>1.0 sec) electricalevents. If the inter-probe interval is selected to be one second, DSP(rRMS) data appears, via digital scroll, in the second probe window thesame as it appeared in the first window, but one second later. Asoftware algorithm may detect a supra-threshold event in the first probewindow and re-analyze it one second later in the second probe window. Atthe time of detection of a supra threshold event in the second window,the activity in both windows is compared. If there Is no supra thresholdactivity in the first probe window, the activity appearing In the secondwindow had a duration of less than one second. If supra-thresholdactivity occurs simultaneously in both windows, the duration of theactivity observed In the second probe window is taken as equal to orgreater than one second. The inter-probe interval may be varied as ameans to distinguish responses greater than or less than the selectedinterval value, This additional strategy may further enhance the abilityto discretely select which events are to be analyzed by the artifactdetection logical circuit for feedback to the operating surgeon. AsIndicated previously, small amplitude responses, which distribute toonly one recording channel, and brief (1.0 sec) repetitive EMG responsesmay be analyzed relatively inaccurately by the presentartifact-detection strategy. During surgical procedures, single or weakresponses may be of important localizing value.

[0053] Optionally, additional DSP analysis is used to help distinguishlocalizing non repetitive EMG activity from electrical artifacts andbrief epochs of repetitive EMG activity. For example, supra-thresholdelectrical events can be captured into a stable buffer for DSP analysis.Additional mathematical treatment of rRMS data is employed foracquisition of additional features which are distinct from thoseselected for general threshold detection purposes. Repetitive EMGactivity typically exhibits a more even power distribution thannon-repetitive EMG activity. A comparison or ratio of peak and averagepower distinguishes the two activities. The values of peak and averagepower required to achieve a reliable fractionation are altered withinthe software and different initial mathematical treatment of DSP data,such as fast Fourier transform, may be useful in separating artifactsand EMG. However, additional DSP methods are presently considered to beless reliable than the use of “Intelligent” and “non-intelligent”distributions for distinguishing artifacts and EMG activity. Their useis preferably user enabled and software algorithms are capable ofperiodic updates in order to take advantage of the accumulation ofempirical data.

[0054] In one embodiment, the output of an additional DSP analysis isavailable as an additional input to the logic circuit, involved withdetecting “intelligent” and “non-intelligent” distributions of suprathreshold events. Alternatively, outputs of the logic circuit and theadditional DSP may provide input to a separate (third) controller,containing software algorithms for decision making. In either case, thesoftware algorithms may Incorporate a hierarchy or system of assigningemphasis or “weight” to various inputs. For example, if electricalactivity is detected simultaneously in the artifact detection electrodewith a supra-threshold event detected in an “non-intelligent” location,this input suggests that the supra-threshold event was an artifact andmay override any other DSP input to the contrary. Alternatively, ifthere was no simultaneous activity seen in the non-intelligentelectrode, but a supra-threshold event is observed in only one of threeor four active “intelligent” channels, the confidence that this is atrue EMG response may be considerably less assured. In such an instance,a hierarchy may be constructed within the decision making softwarealgorithm that may allow certain DSP data to override the initial“verdict,” based upon spatial distribution.

[0055] Turning to another aspect of the present invention, as notedabove, quantitative measurements of nerve function in intraoperativemonitoring are relatively cumbersome and require involvement oftechnical personnel to change stimulator settings and various recordingparameters in order to acquire, analyze, display and store data. Theapplicant has noted that there are not many types of quantitativemeasurements regarding nerve function assessment, however, and thatthreshold and peak-amplitude measurements are the most widely used. Theapplicant has also discovered that paired stimuli pulses areparticularly effective when assessing nerve fatigue. Operating surgeonsusually have specific preferences regarding the type of quantitativedata to be collected and analyzed during the course of a given surgicalprocedure, so there is little need for “on-the-fly” flexibility in theoperating room (OR) when performing quantitative data collection.

[0056] Quantitative data on nerve function is mainly acquired throughthe use of an electrical stimulus probe, which provokeselectromyographic responses for quantitative analysis.

[0057] The inventor has observed that surgeons use the stimulus probedifferently for locating and “Mapping” than for quantitative analysis ofthe functional status of nerves of interest. Temporal aspects ofstimulus probe use can be monitored by the tissue contact detectioncapability within the digital stimulator as described previously. Asignal is generated in the stimulator that relates to the period ofcontinuous contact of the stimulator probe with patient tissue. Thesignal continues as long as continuous tissue contact is maintained andis delivered to a system controller, which is able to initiate multiplepredetermined sequential and parallel operations within the nerveIntegrity monitor. These operations relate to delivery of preprogrammedstimulus sequences and to the acquisition, analysis, display andarchival storage of EMG data. Whether the predetermined operations areinitiated or completed depends upon the duration of continuous tissuecontact. For example, if the duration continuous tissue contact is lessthan a preselected period of approximately one or two seconds, thecontroller will maintain the operational status of the nerve Integritymonitor in the “search” mode. However, if the duration of continuoustissue contact exceeds the preselected time period, the stimulator orcontroller may alert the surgeon with an indicator tone and controllerwill automatically change the operational status of the nerve integritymonitor to a quantitative assessment mode. The Indicator tone may alsobe designed or configured to signify whether or not adequate currentand/or stimulator circuit impedance has been achieved, as an indicationof quality assurance.

[0058] From the time of tissue contact detection, a digital clock isinitiated, controlling a preset sequence of events through a controllerInterface. For the purposes of this description, the period ofcontinuous tissue contact of the stimulus probe is termed the “dwell” or“dwell time”, and the series of preselected operational changes provokedby the “dwell” is termed, the “Tissue Contact Initiated Event SequencingTime line” or “TCI-Time line”. The control method to be described isdesigned for use with the main stimulus probe (e.g., stimulus output #1)and may be used to control all functions of the nerve integrity monitorin a preselected fashion. The described methodology need not be limitedto medical applications, in that the use of any probe, where its periodof dwell can be measured, may be similarly configured to controlmultiple functions. The following description involves the preferredembodiment, although many possible sequence strategies are availablethrough the TCI-Time line:

[0059] Through the associated controller and controller interface, theonset of dwell will cause the artifact-detection circuit to be suspended(“defeated” ) throughout its duration and a preset pattern of stimuluspulses, the intensity of which is determined by front panel controls,will be delivered through the stimulator probe for locating and“mapping” the physical contour of the nerve of Interest. After apreselected dwell time of approximately one second, front panel controlof stimulus parameters is defeated, the pattern of stimuli is changedfrom single pulses to alternating paired pulses with single pulses, theintensity of which is somewhat greater (supra-maximal), and the provokedEMG responses are digitized and individually captured into stablebuffers. If the dwell is interrupted before a dwell of 2 seconds, theTCI-Time line is inactivated, the artifact-detection circuit Is enabled,the stable buffers are cleared of captured signal and pulsed stimuli areno longer delivered through the stimulus probe. After a 2 secondpreselected period of dwell, the controller and associated interfaceinitiate a signal processing sequence, where the captured responses Instable buffers are analyzed by averaging the single and paired responsesseparately and computing the difference between the paired and singleresponse by digital subtraction. The magnitude of the single anddigitally subtracted responses are computed and compared. A scalar valuerelating to a ratio of the magnitudes of the digitally subtractedresponse and the single response is stored in a spreadsheet against theabsolute or lapsed time (of the operation) and is displayed by CRToutput automatically or upon an input “ request” by the operatingsurgeon. The stable buffers used in these computations are automaticallycleared at completion. The above computational operations occur inparallel to the following:

[0060] After a 2 second preselected period of dwell, the controller andinterface defeat front panel control of stimulus parameters and alterthe stimulus delivery pattern to a series of single pulses of varyingintensity. The controller and interface direct the provoked EMGresponses to be captured individually into stable buffers. If the dwellis interrupted prior to completion of the stimulus sequence, theTCI-Time line is discontinued, the sequence of stimulator pulses isdiscontinued, the stable buffers are cleared of captured signal, theartifact-detection functions are enabled and stimulus parameters arereverted to front panel controls. However, interruption of the dwellafter 2 seconds does not Interfere with the completion of the paralleloperations described above regarding the mathematical treatment of EMGactivity-provoked by single and paired stimulus pulses.

[0061] If the dwell is continued (after 2 seconds), then until thestimulus sequence is completed, the stimulator or TCI-Time linecontroller delivers a second indicator tone and the controller andinterface initiate a series of operations to generate a scalar value ofresponse threshold. Each individually captured EMG response is analyzedfor power content (peak or average), the scalar value of which is storedin a spreadsheet in conjunction with the stimulus intensity used toprovoke it. The spreadsheet data relating to all stimulus intensitiesand corresponding responses is used to compute (or estimate) thestimulus intensity in milliamps (mA) at which half-maximal responsemagnitude (power) occurred. This scalar value (in mA) is then defined asthe “response threshold” and is applied to a spreadsheet againstabsolute or lapsed time of the surgical procedure. The scalar value or agraphical plot of threshold versus operative time may be displayedautomatically by CRT screen or displayed upon request by input suppliedby the operating surgeon. These computational operations are carried outin parallel with progress of the dwell and may reach completionconsiderably after the dwell has been interrupted.

[0062] As described, the “TCI-Time line” Is a multidimensional controlalgorithm or device utilizing information spanning both time and space.The continuous tissue contact dwell serves to initiate various series ofoperations through the TCI-Time line controller and interface. Theseoperations may include simple or complex stimulus delivery paradigms,and corresponding data acquisition, analysis, display and archivalstorage procedures. The stimulation sequences and data handlingalgorithms proceed along different time lines, as per pre-programmed,parallel (processing) software algorithms. As long as the dwellcontinues, these operations proceed to completion in sequence.Alternatively, interruption of the dwell aborts all subsequentinitiation of events along the dwell, but may allow some of thepreviously initiated events to reach completion as described above. TheTCI-Time line controller directs operational events in differentlocations within the nerve integrity monitoring device. Production ofstimulus pulses occurs in the stimulator portion of the monitor, whiledata acquisition, analysis, display and storage may occur in differentlocations, such as on the memory of a PCI card, CPU RAM memory or a harddrive. Thus the present TCI-Time line control system must account formultiple time dimensions and multiple locations within the monitoringdevice.

[0063] Detection of tissue contact is preferably achieved by continuousstimulator circuit impedance measurement or continuous measurement ofcurrent flow with use of a separate sub-threshold current delivereddownstream from actual pulsed stimuli to the patient. Either of thesemethods will allow the detection of the temporal pattern caused bytapping the stimulator probe two or three times onto patient tissue(away from Important structures) as a means of providing additionalinput to the controller through the tissue contact detection circuit. A“double” or “triple” tap of the stimulus probe may be preselected foraltering the normal operation of the controller, such as initiating adisplay of previously stored data as a “time trend.” That is, a “doubletap” command may provoke the controller to display a time trend of ameasured parameter, such as response threshold. The scalar value ofstimulus intensity (mA), where the response threshold is achieved, isplotted against time (duration of the operation) to give the surgeon aclearer impression of how the nerve of interest has responded throughoutthe surgical procedure.

[0064] Optionally, the control capabilities of the TCI-Time line areused for analyzing and storing data derived from detection of suprathreshold events. Supra threshold events may transferred from stablebuffers, described previously with regard to “additional DSP” analysisof supra threshold events, and converted to file format for archivalstorage. The file of the digitized signal, its scalar DSP values (e.g.,peak and average rRMS), and its channel number (or identity) may bearchived (as in a spreadsheet) against the absolute or lapsed(operative) time of its appearance for later (off-line) retrieval. Suchcapabilities improve the ability to “tune” DSP parameters for greateraccuracy in detecting appropriate events for analysis, for alerting theoperating surgeon and for distinguishing artifacts from true EMG.

[0065] Preferably, audio and video capture devices are integrated intothe system to perform audio and video data capture functions. Anindependent method of distinguishing artifact and EMG supra thresholdevents is to interpret events in the context of the surgical procedure.If the supra threshold event occurred exactly at the time of a surgicalmanipulation, it may be interpreted as a mechanically stimulated (hencenon-repetitive) EMG event. Alternatively, if the event appears to occurindependently of surgical manipulations it is interpreted as eitherartifact or non-localizing (repetitive) EMG. Relatively brief (3-5seconds) periods of digitized audio signal of the sound delivered to thesurgeon through the loudspeaker in the nerve integrity monitor anddigitized video of the surgical procedure, from a (microscope or handheld) camera monitoring the surgical field, is adequate to interpret the“context” of a supra threshold event. Audio and video signal may bedigitized and held in FIFO “scroll” buffers within the nerve integritymonitor. For investigational purposes, the logical circuits used fordetection of supra threshold events may send a signal to the TCI-Timeline controller when certain preselected supra threshold events aredetected; the signal provokes the TCI-Time line controller to cause thecapture of digitized audio and video for an interval starting 2-4seconds before and ending one second after the onset of the suprathreshold event. The captured audio and video can then be converted tofile form (*.avi, *.mpg or equivalent) and archived along with thesignal data mentioned above. Such capability tremendously facilitatesevaluation (validation) of various methods of event (artifact and EMGresponse type) detection for accuracy and effectiveness.

[0066] With the present control system, temporal aspects of stimulusprobe use can be made to control an entire quantitative analysisparadigm in a pre-programmed, preset manner, based upon the needs of theuser. This will involve a mix of sequential and parallel operations andsmooth operation is dependent upon a seamless digital CPU interface forcontrol of data acquisition, analysis and display, preferably in awindows based software system. The algorithm steps or command sequencesand interrupt interpretations are stored on non volatile memory, such asEEPROM or “flash memory,” providing fast online operation in acontroller which is readily reprogrammed or modified off-line byCPU-interface. At present, the prevailing standard digital interface isthe Peripheral Components Interface (PCI); it is to be understood thatfuture developments may provide equivalents to the PCI standard.Accordingly, the following discussion is a description of but oneexemplary embodiment which happens to include a PCI circuit card.

[0067] The enhanced or “complete” neurophysiological monitoring systemconsists of the basic monitoring unit, a processor including a CPU(e.g., an Intel Pentium® brand microprocessor) and a PeripheralComponents Interface (PCI) circuit card. The CPU interfaces with thebasic monitoring unit through the PCI for both off-line and on-lineoperations. Digitized signals from the basic monitoring unit arecontinually delivered (e.g., via an optical transmission link) to thePCI card, which continually routes them to temporary scroll buffers.When triggered by the tissue contact initiated (TCI) Time line or bydetection of evoked EMG responses, recorded signal events are“captured,” along with time, data channel identification and otherrelevant information. The captured signals are held in a stable bufferfor DSP manipulations (e.g., Fast Fourier Transform (FFT) frequencyconversion) and for conversion to a selected file format. A scrollbuffer is a first-in-first-out (FIFO) image buffer storing the mostrecent waveform segment; the stored segment has a selected duration(e.g., approx. 2-10 seconds). A stable buffer (or bin) is also an imagebuffer but only holds discrete supra-threshold waveforms or events, andso effectively ignores the waveform trace between events; the stablebuffer holds waveforms of selected durations or epoch lengths (e.g.,approximately 1 second).

[0068] The PCI interface includes the scroll buffers and the stablebuffers containing captured signal data for quantitative facial nervesignal assessment. Associated DSP circuitry is located on a PCI circuitboard. There must be a relatively generous number of stable buffers (orbins) available to separately capture one or more given EMG events onmultiple channels and to capture individual responses relating tostimuli of differing parameters (intensities). Additional buffer spacesor bins must also be available for digital subtraction functions, wherea “third” bin stores the computed difference between two others forfurther quantitative analysis. The total number of bins must be adequateto handle a variety of analysis algorithms.

[0069] There must also be consideration for how signals occurringsimultaneously or nearly simultaneously will be processed. Theindividual bin size must be adequate to store a large number of samples,thereby providing adequate waveform fidelity and the sample rate ortime-base must be high enough to capture signals with the requiredaccuracy.

[0070] The processor includes a motherboard having a CPU for acquisitionof scalar data from the DSP circuits on the PCI-card and for datapresentation functions such as spreadsheeting and graphing (e.g., usingLotus® or Excel® spreadsheet programs) and for controlling the display.The processor is preferably also configured to create, tag and bundledata files from the temporary stable buffers on the PCI card. Image datafiles are preferably bundled with corresponding “captured” audio andvideo files (from separate video and sound cards) and then transferredinto permanent storage in appropriate locations on the processor harddrive for later review. Off-line, saved data is readily re-loaded intotemporary stable buffers to permit the surgeon to review or re-analyzedata to observe the effectiveness of artifact recognition and nervefunction assessment.

[0071] Thus, the system delegates DSP functions to various componentsfor rapid performance of mathematical operations and display of data.Complex stimulation paradigms are initiated by a digitally controlledstimulator, based upon temporal aspects of tissue contact by the mainstimulus probe. The digital stimulator (or the controller executing theTCI-Time line algorithm) sends simultaneous signals through thePCI-interface to direct data to the appropriate buffers (or bins) foron-line analysis. Additional signals, either from the basic monitoringunit or internally generated on the PCI by pre-programmed algorithms,initiate pre-set data-display and data storage algorithms. Six to twelvedifferent stimuli and a corresponding number of storage buffers may beemployed for threshold detection. Alternating paired and single pulseswill require at least three bins. One each for binning responses evokedby paired and single pulses, and a third for holding computed digitalsubtraction data. Optionally, within the two bins for single and pairedresponses or by combining the results of separate bins, repetitiousresponses may be used to compute a signal “average” for single andpaired responses. The respective averages may be used to compute thedigital subtraction data for the “third” bin.

[0072] Complete control over on-line operations of the intraoperativeneurophysiological monitor of the present invention can be achievedthrough the use of the TCI-Time line and is preferably set up off-lineusing keyboard and mouse input devices through a standard personalcomputer operating system such as Microsoft Windows® software.

[0073] A preferred embodiment is that all changes made by off-line inputprocedures are transferred to the main unit of the nerve integritymonitor and “burned in” to non-volatile (EEPROM or flash) memory. As aresult, the information transferred will be protected from spuriousvoltage spikes and accidental unplugging. This Is distinct from priorart methodology, where off-line changes are stored in volatile memory,which may be susceptible to spurious voltage spikes and accidentalunplugging of equipment.

[0074] Additional on-line flexibility is afforded through use of simpleinput devices which are convenient and easy to use, but not ascomprehensive as the keyboard and mouse combination; in one embodiment,the stimulus probe is used as a pointing device for inputs to thecontroller. During surgery, or when “on-line”, an electrical stimulusprobe is preferably employed as a convenient controller input device andthe TCI-Time line algorithm controls most on-line system operations,including which data are displayed to the operating surgeon on the CRTscreen display, however, the surgeon may periodically want to seeadditional information, such as a display of a measured parametergraphed as a function of time, over course of the procedure. Thestimulus probe provides a convenient and simple input device forinitiating such requests, since the surgeon is likely already holdingthe probe, and so need not put the probe down to use a keyboard, or thelike.

[0075] The TCI-Time line algorithm is triggered upon detection of tissuecontact by the electrical stimulus probe. Tissue contact detectionincludes probe signal current flow or impedance-change detection.

[0076] In addition to providing an indication of presence or absence oftissue contact, the tissue contact detection apparatus is configured torecognize specific signatures, such as a “double tap” or “triple tap” ofthe-stimulus probe against non-sensitive patient tissue within thesurgical field. The detection of these predetermined signatures can beused to provide additional online input to the TCI-Time line controller.When such a pattern is detected, a separate signal is sent to theTCI-Time line controller for initiation of context sensitive,predetermined commands, a sequence analogous to a “double click” of astandard mouse when pointing to an icon in a Windows® compatibleprogram. The identity of these commands are changeable, depending uponthe monitoring context of the request; context is provided by theTCI-Time line algorithm. If the “double-click” occurs before thecompletion of a TCI-Time line controlled operation, the request isinterpreted differently than for a double-click occurring aftercompletion.

[0077] The tapping pattern can differ among different users, in orderfor the tapping pattern of a given user is recognized, a setup algorithmincludes an adjustment method allowing the user to input his or herindividual tapping pattern. Recognition of tapping patterns may beperformed by “default” recognition settings within the tissue contactdetection circuitry. However, because the temporal aspects of tappingmay vary significantly among individual surgeons, the preferred systemallows an individual surgeon's tapping signature to be captured forlater recognition. It is preferred that this is performed early in thesurgical procedure, before critical stages. For this procedure, a frontpanel or foot pedal switch is depressed, immediately after which thesurgeon performs a “double tap” or “triple tap” signature. The patternof impedance change or current flow change detected by the tissuecontact detection circuitry is stored and used as a template forrecognition of similar “signature” patterns at a later time.

[0078] Also, when the double- or triple-tap input command is used, asound sample or audible annunciation is preferably activated to indicatethat the intended command has been successfully communicated. The soundsample might can be any form of effective audible feedback to the user(e.g., a sound of a standard mouse double-click or triple-click).

[0079] After completion of a TCI-Time line controlled, pre-programmedstimulus sequence with corresponding quantitative data display, thealgorithm preferably includes program steps for detecting a stimulusprobe double-tap and, in response, displaying all similar measurementsobtained from the beginning of the procedure (e.g., traced as a waveformshowing voltage as a function of time), wherein a time-trend ofstimulation threshold can be observed to detect a significant injury inprogress. Similarly, after supra-threshold detection of an EMG response,the algorithm may include “if then” condition detection program stepswherein detection of a “double tap” is the input causing a display ofthe IDSP data for that response or for a display of a DSP-derivedparameter, such as root mean square (RMS) power, as a function of time.Such a trend may show a loss of signal power over the course of theprocedure and may indicate a fatigue trend in the nerve underobservation in response to ongoing mechanical manipulations.

[0080] A simple input device used in conjunction with the TCI-Time linealgorithm alternatively includes two or three button operated switchesaccessed from a cylindrical handle. The two button configuration used ina manner similar to setting of a watch; one button selects options froma menu displayed on the nerve integrity monitor and the other button isused to choose a user preference or selection from the menu of options.Alternatively, a three-button input device provides more flexibilitywith forward and backward movement through a menu or series of menus,since the buttons could be used to scroll up, scroll down or selectoption, respectively. The simple input device is readily kept sterile onthe field and its simplicity allows rapid data or control input and easeof use. Such a device does not require the use of the stimulating probe.

[0081] The above described simple devices for on-line use provide inputthrough the monitoring system controller digital interface, rather thanthrough a serial port of the host computer. Off-line operations,controlled by keyboard and mouse, preferably operate through mouse andkeyboard ports on the controller CPU.

[0082] As discussed above, the intraoperative neurophysiologicalmonitoring system also includes an enhanced method and algorithm fordetecting or thresholding non-repetitive EMG events or activity (such asthe short duration pulses indicative of EMG activity) as distinguishedfrom repetitive EMG activity, even when the non-repetitive EMG eventsare sensed simultaneously with the repetitive EMG events, in which casethe waveforms are superposed upon one another. The enhanced thresholddetection algorithm includes the steps of buffering or storing acontinuous series of samples of the sensed EMG waveforms from one ormore sensing electrodes; the buffered waveform is processed by runningthe stored waveform samples serially through spaced probefirst-in-first-out (fifo) sampling windows of selected duration andhaving a selected temporal spacing therebetween; in the preferredembodiment, the probe sampling windows have a duration in the range of0.25 seconds to 0.5 seconds and the beginning of the first probesampling window is temporally spaced at one second from the beginning ofthe second probe sampling window. The algorithm passes the storedwaveform samples serially through first probe sampling window and thenthrough the second probe sampling window. As the stored waveform samplespass through each probe sampling window, a scalar value corresponding tothe rectified RMS (rRMS) power of the waveform is generated. Thealgorithm continuously computes a threshold value by subtracting theinstantaneous value of the second probe sampling window rRMS power fromthe first probe sampling window rRMS power; the continuously generatedresults of this computation are readily plotted as a threshold valuewaveform. Since the algorithm passes the stored waveform samplesserially through the first probe sampling window and then through thesecond probe sampling window, a non-repetitive EMG activity will producea threshold value waveform having a first, positive going pulse having awidth approximating the duration of the non-repetitive EMG activity(corresponding to the first probe sampling window rRMS power) and then asecond negative going pulse having the same width (corresponding to thesubtracted second probe sampling window rRMS power).

[0083] A repetitive EMG activity having a duration longer than theselected (1.0 second) spacing between the probe sampling windowsproduces a threshold value waveform having only one positive going pulsehaving a width approximating the duration of the interval beginning atthe start of the first probe sampling window and ending at the start ofthe second probe sampling window (in the present example, a duration ofone second). For a stored waveform having a non-repetitive EMG activitysuperposed on a repetitive EMG activity, as above, the algorithm willproduce a threshold value waveform having a first one second long pulseincluding a second positive going pulse having a width approximating theduration of the non-repetitive EMG activity (corresponding to the firstprobe sampling window rRMS power) and then a second negative going pulsehaving the same width as the non-repetitive EMG activity (correspondingto the subtracted second probe sampling window rRMS power).

[0084] Whenever the enhanced threshold detection algorithm produces athreshold value waveform including a first positive going pulse followedby a second negative going pulse, there is an indication that a brief(e.g., <1.0 sec) response has occurred which may be either localizingnon-repetitive EMG or artifact. Detection of such an event provokes theartifact-detection circuitry to evaluate its spatial distribution among“intelligent” and “non-intelligent” electrodes and (optionally)additional DSP algorithms in order to determine its status as anartifact or (localizing) EMG event. The surgeon is then prompted with anappropriate audible and (optionally) visual annunciation.

[0085] Another aspect of the present invention is a method for reducingirritating an distracting noise from repetitive EMG activity madepossible by the enhanced threshold detection strategy described above.Data from all (and exclusively) “intelligent” EMG channels is digitizedand monitored by the enhanced threshold detection circuit, employing twoprobe windows as described, with an inter-probe interval ofapproximately one second. By DSP, the average rRMS is continuouslycomputed for both windows and the scala value is referenced againstelectrical silence. With the two probe window strategy, if only onewindow is active at a time, the duration of a supra threshold event mustbe less than the inter-probe interval. If both windows are activesimultaneously, the duration is equal to or greater than the inter-probeinterval. Since the vast majority of non-repetitive activity is lessthan one second in duration, an inter-probe interval of one second isable to effectively distinguish repetitive and non-repetitive responses.Repetitive responses are detected when both probe windows aresimultaneously active. In the “automatic” squelch embodiment, the scalarvalues of average rRMS derived from the two probe windows arecontinuously scanned by a software comparator constructed innon-volatile memory. The comparator is configured to compare ongoingaverage rRMS values against a user preselected threshold value. If thethreshold value is exceeded in both probe windows, a signal is generatedwhich activates a muting switch to eliminate that particular channelfrom the audio (loudspeaker) signal to the operating surgeon. If otherchannels reach supra threshold levels of continuous repetitive EMGactivity, more channels may be muted, except the last (quietest)channel. That is, no matter how much repetitive activity, at least one“intelligent” channel is preserved for continuous audio display of EMGsignals to the operating surgeon. When the average rRMS values of bothwindows decrease below threshold levels, the muting switch isautomatically disabled.

[0086] In an alternative manual squelch embodiment, the muting functioncan be enabled manually. Some surgeons may prefer to decide on a “caseby case” basis, when to begin muting offending EMG channels. Whenbothered by persistent repetitive EMG activity, the surgeon may requestthat a nurse or technician depress a momentary push-button switch,conveniently located on the front panel of the nerve integrity monitor.With activation of the push button switch, all (except the quietest)channels with supra threshold levels of repetitive EMG activity aremuted from the audio signal to the surgeon. As with the previous“automatic” embodiment, once the activity has quieted to sub-thresholdlevels, the audio output is automatically re-enabled. It is preferredthat the surgeon be given the option of automatic and manual operationby a simple front panel control selections.

[0087] The above and still further objects, features and advantages ofthe present invention will become apparent upon consideration of thefollowing detailed description of a specific embodiment thereof,particularly when taken in conjunction with the accompanying drawings,wherein like reference numerals in the various figures are utilized todesignate like components.

BRIEF DESCRIPTION OF THE DRAWINGS

[0088]FIG. 1 is a block diagram of the intraoperative neurophysiologicalmonitoring system digitally controlled stimulator impedance and currentflow detection circuit elements, in accordance with the presentinvention.

[0089]FIG. 2 is a flow diagram illustrating the impedance detectionalgorithm for detection of tissue contact, for use with the monitoringsystem stimulator impedance detection circuit of FIG. 1, in accordancewith the present invention.

[0090]FIG. 3 is a flow diagram illustrating the impedance detectionalgorithm for detection of tissue contact, for use with the monitoringsystem stimulator current and impedance detection circuits of FIG. 1, inaccordance with the present invention.

[0091]FIG. 4 is a block diagram of the intraoperative neurophysiologicalmonitoring system digitally controlled stimulator current flow detectioncircuit, in accordance with the present invention.

[0092]FIG. 5. is a flow diagram illustrating the current flow detectionalgorithm for detection of tissue contact, for use with the monitoringsystem stimulator current flow detection circuit of FIG. 4, inaccordance with the present invention.

[0093]FIG. 6 is a top view of an artifact detection electrode inaccordance with the present invention.

[0094]FIG. 7 is graphical representation of a pre-programmed set ofelectrical stimulus pulses of varying intensities used in theintraoperative monitoring of responses to stimulus pulses, incombination with a flow diagram illustrating the steps in the TCI-timeline algorithm and the context sensitive interrupt commands forcontrolling whether the TCI time line algorithm is aborted or completed.

[0095]FIG. 8 is a block diagram of the intraoperative neurophysiologicalmonitoring system TCI-Time line algorithm controlled impedance andcurrent flow detection circuit, in accordance with the presentinvention.

[0096]FIG. 9 is a block diagram of the intraoperative neurophysiologicalmonitoring system TCI-Time line algorithm controlled current flowdetection trigger circuit, in accordance with the present invention.

[0097]FIG. 10 is a set of related waveform traces, plotted as a functionof time, illustrating first and second probe sampling windows each of aselected duration and temporally spaced at a selected inter-probeinterval.

[0098]FIG. 11 is a waveform trace, plotted with voltage as a function oftime, illustrating a non-repetitive EMG activity.

[0099]FIG. 12 is a waveform trace, plotted with voltage as a function oftime, illustrating a repetitive EMG activity.

[0100]FIG. 13 is a set of related waveform traces, plotted with voltageas a function of time, illustrating a non-repetitive EMG activitysuperposed on (or occurring and sensed simultaneously with) a repetitiveEMG activity.

[0101]FIG. 14 is a set of related waveform traces, plotted with voltageas a function of time, illustrating a sensed non-repetitive EMG signaltemporally situated between first and second probe sampling windows ofselected durations, and the Rectified RMS power derived from thedifference detection algorithm for first and second probe samplingwindow durations.

[0102]FIG. 15 is a set of related waveform traces, plotted with voltageas a function of time, illustrating a sensed repetitive EMG signal of aduration including first and second probe sampling windows of selecteddurations, and the Rectified RMS power derived from the differencedetection algorithm, for first and second probe sampling windowdurations.

[0103]FIG. 16 is a set of related waveform traces, plotted with voltageas a function of time, illustrating a sensed non-repetitive EMG signaltemporally situated between first and second probe sampling windows andsuperposed on a sensed repetitive EMG signal of a duration including thefirst and second probe sampling windows, and the Rectified RMS powerderived from the difference detection algorithm, for a selected probesampling window duration.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0104] Referring specifically to FIG. 1 of the accompanying drawings, anintraoperative neurophysiological monitoring system 20 includesdigitally controlled stimulator impedance and current flow detectioncircuit elements for use during intraoperative neurophysiologicalmonitoring and preferably in conjunction with a tissue contact initiatedevent sequencing time line algorithm (TCI-Time line) for control of dataacquisition, analysis, display and storage. A current source 20 isconnected with parallel inputs to three electronic switches(“current-gates”) CG-1,CG-2, and CG-3, 24,26 and 28; although separatecurrent sources for each stimulus output may be employed, a singlecurrent source is shown here. Each stimulus output includes astimulus-controller (e.g., SC-1) 30 controlling the intensity, durationand temporal patterns of delivered stimulus-pulses. The controller is,in turn, connected with controlled by and responsive to adigital-interface (e.g., DI-1) 32 which is in turn connected to a CPU(not shown) and a comparator (e.g., C-C1) 34. Each stimulus output alsoincludes a current detection circuit (e.g., CD-1) 36 and a stimulusisolation unit (e.g., SIU-1) 38. Each stimulus output includes anelectronic switch (e.g., 24) which is responsive to and driven bytissue-contact detection. The present design uses impedance-detection asthe means to detect tissue-contact. Switches 24, 26 and 28 are kept inthe open circuit position until tissue-contact is detected at one of thecathode terminals at the output. Tissue contact produces a signal fromthe corresponding impedance-detection circuit to close the electronicswitch, the probe for which is in contact with tissue, and open circuitsthe other switches. Preferably, switches 24, 26 and 28 are configured sothat only one switch (e.g., 24) can be closed at a time. Current flow ismeasured for each stimulus output by a current-flow detection circuit(e.g., 36), and the output of circuit 36 drives the digital indicationor read out of current-flow for the corresponding stimulus output. Theoutput signal of the current detection circuit 36 is responsive tomeasured current flow and is compared against the “user-intended”current level by comparator circuit 34.

[0105] If the current value falls within predetermined limits (90-95%),comparator circuit 34 outputs an “enable” signal, to be used to triggeran “adequate-current” speech sample or tone, thereby providing audiblefeedback for the surgeon, the detection of the enable signal is atriggering event for execution of the TCI-Time line algorithm in the CPUand Digital interface 32. Each stimulus output includes a digitalinterface (e.g., 32) storing various stimulus paradigms, which areinitiated in a pre-programmed fashion (as by the TCI-Time linealgorithm) upon tissue-contact detection. Digital-interface 32 alsodirects data capture, analysis, display and storage in a pre-programmedfashion per the TCI-Time line. The interface 32 consists of twocomponents, one of which is located in a system main monitoring unit,the other of which is located in a PCI-bus slot in a system computerDigital interface 32 employs stimulus controller 30 which shapes thecurrent provided from the current-source 22 into stimuli ofpre-programmed intensity, duration, and having a preselected temporalpattern. The stimulus controller 30 is driven by digital interface 32,which stores stimulus-paradigms in non-volatile memory and initiates thestimulus paradigms as pre-programmed within the TCI-Time line algorithm.Digital Interface 32 also controls functions relating to dataacquisition, analysis, display, and storage through its connection withthe CPU. For stimulus output #2, #3 or both, digital interface 32 may beconfigured to input measured values of stimulus circuit impedance andmake pre-programmed adjustments of stimulus intensity, based uponimpedance values. It is anticipated that stimulus output #1 will be usedwith a flush-tip stimulus probe for which such an application is notnecessary.

[0106] In FIG. 1, impedance detection circuit ID-1, 40 is included toprovide an indication of tissue-contact and to measure nominal stimuluscircuit impedance. Detection of tissue-contact is used to initiate theTCI-Time line algorithm and measurement of impedance is used to providea “quality-check” of the stimulus-circuit integrity and provide a meansof adjusting stimulus intensity to the level of current shunting. Forimpedance measurement, impedance detection circuit 40 provides a small,sub-threshold signal that is detected. The patient connections for theimpedance-detection circuit 40 is electrically or optically isolatedfrom the line-powered circuitry by connection through circuit isolationelement I-I1, 42, and is preferably connected to a comparator 44 whichreceives the output of impedance detection circuit 40 and computesscalar representations of measured stimulus-circuit impedance.Comparator 44 provides an output for use by the digital interface 32 todrive the various data-handling operations and preprogrammed stimulusintensity adjustments.

[0107] Turning now to FIG. 2, a flow diagram illustrates the impedancedetection algorithm for detection of tissue contact, for use with themonitoring system stimulator impedance detection circuit of FIG. 1. Onceprobe 50 is brought into contact with the tissue of a patient, the probeimpedance (which is initially an open circuit impedance at the probetip) is reduced to the measured tissue impedance and switch 24 is closedto provide stimulus current. Substantially simultaneously, the TCI-timeline algorithm is initiated and a light emitting diode (LED) isilluminated, thus providing an indication of “appropriate” tissuecontact impedance. Stimulus current flows through probe 50 and the nervetissue and the current flow is detected in current detection circuit 36;if the detected current is of the appropriate magnitude, an LED isilluminated, thus providing an indication of “appropriate” current flow.Once the surgeon lifts the probe and interrupts tissue contact, theincreased, open circuit impedance is detected, and switch 24 is opencircuited in response, while the impedance detection circuit 40 isactivated and the TCI-Time line algorithm is reset.

[0108]FIG. 3 is a flow diagram illustrating the impedance detectionalgorithm for detection of tissue contact, for use with the monitoringsystem stimulator current and impedance detection circuits of FIG. 1.Once probe 50 is brought into contact with the tissue of a patient, theprobe impedance (which is initially an open circuit impedance at theprobe tip) is reduced to the measured tissue impedance and switch 24 isclosed to provide stimulus current. Substantially simultaneously, theTCI-time line algorithm is initiated and a light emitting diode (LED) isilluminated, thus providing an indication of “appropriate” tissuecontact impedance. Stimulus current flows through probe 50 and the nervetissue and the current flow is detected in current detection circuit 36;if the detected current is of the appropriate magnitude, the impedancedetection circuit 40 is open circuited and an LED is illuminated, thusproviding an indication of “appropriate” current flow. Once the surgeonlifts the probe and interrupts tissue contact, current flow stops andthe lack of current flow is detected with current detection circuit 36,whereupon switch 24 is open circuited in response, while the impedancedetection circuit 40 is re-closed and the TCI-Time line algorithm isreset.

[0109] The intraoperative neurophysiological monitoring system 20comprises a stimulator that preferably includes a nerve integritymonitoring instrument having multiple independent stimulus outputs toprovide optimal preset stimulus output parameters for more than oneprobe type, thereby allowing all probes to be connected at the beginningof the case and used as needed, without delay or confusion related toswitching and intensity setting changes. Independent, electricallyisolated outputs also eliminate parallel connections among stimulusprobes and possible current leakage between probes. In the exemplaryembodiment of FIG. 1, three stimulus outputs include a monopolar probe50, a bipolar probe (not shown) and an electrified instrument (notshown), all three simultaneously connected.

[0110] For the purposes of nerve integrity monitoring, electricalstimulus probe 50 is used for locating and defining the contour of thenerve of interest. During “mapping” procedures, the stimulus probe 50 ismoved about the surgical field or along the nerve contour in smallcontrolled steps, during which probe 50 is in continuous contact withtissue, usually for less than one or two seconds. Alternatively, duringquantitative measurements of nerve function, probe 50 may be applied tothe nerve continuously for a few or several seconds allowing capture ofelectromyographic activity for analysis. Thus, if probe 50 is in contactwith tissue for less than one or two seconds, it may be taken that thesurgeon is simply locating or mapping the contour of the nerve ofinterest. If continuous tissue contact exceeds one or two seconds, thesurgeon's intent is likely to be otherwise, such as for quantitativemeasurements. Further, if the stimulus probe 50 is tapped twice or threetimes onto patient tissue, the temporal pattern of continuous tissuecontact is different from either of the previous patterns and isconsidered a “request” by the surgeon.

[0111] The present invention incorporates a method of controlling avariety of nerve integrity monitoring functions through detection of theduration of continuous contact of probe 50 with patient tissue.Alternative methods to more accurately detect the temporal pattern ofcontinuous contact of probe 50 with patient tissue include continuousmeasurement of stimulation circuit impedance and measurement of currentflow using a continuous, distinct (second) sub-threshold current,delivered “downstream” from the actual electrical stimulus (e.g., usingthe downstream current source, CS-2, as shown in FIG. 4).

[0112] Returning to FIG. 1, stimulation circuit impedance isaccomplished by using the impedance detection circuit 40 to enable useof stimulator probe 50 as an input device.

[0113] Additional circuit elements 40, 42, 44 are required for impedancedetection, with an additional patient connection electrode 54 (e.g., amonopolar subdermal electrode) having its own isolation circuit 42, andan additional continuous, sub-threshold probe signal (i.e., below thethreshold required for nerve activation) must be delivered through theprobe tip for measurement by the impedance detection circuit 42.

[0114] In an alternative embodiment of the monitoring system 100 (asshown in FIG. 4), a continuous, second sub-threshold current generatedin current source #2, CS-2, 104 is delivered to a stimulus probe 102,downstream from the pulsed current used for actual nerve stimulation.Detection of flow of the continuous current provides more accuratedetection of tissue contact than for pulsed stimulation alone andpermits detecting a “tapping” pattern of the stimulus probe. Continuouscurrent flow detection does not provide as many possible benefits ascontinuous stimulus circuit impedance measurement, but also does notrequire placement of an additional patient electrode and the necessaryisolation circuitry.

[0115] In addition to detecting and responding to a temporal pattern ofcontinuous tissue contact of the stimulus probe, the stimulator of FIGS.1 and 4 are adapted for digital control. Stimulus intensity, pulseduration, and temporal pattern of stimuli presentation are controlledthrough a digital controller having a digital interface circuit 32. Theinterface 32 (and the accompanying CPU) stores pre-programmed stimulusalgorithms or paradigms, preferably in non-volatile memory. The stimulusparadigms are preferably constructed off-line using appropriate stimuluscontrol algorithm development software and is preferably loaded orburned into a non-volatile Read Only Memory (ROM) chip, included withinthe interface. During a monitoring procedure, contact with tissue willtrigger a predefined sequence of events called, for purposes ofnomenclature, a Tissue Contact Initiated (TCI)-Time line, therebyactivating the stored stimulus paradigms in a pre-programmed manner.

[0116] Front panel controls for monitoring system 20 consist of basicstimulus intensity controls. Stimulus, pulse duration and pulserepetition rate are preferably adjusted in a limited manner by recessedDIP-switches or other user-accessed, but less prominent controls. Theremaining stimulator controls are actuated through the digital CPUinterface 32, such as via a PCI bus. As discussed above, monitoringparameters and complex stimulus paradigms are stored via non volatile,programmable memory (e.g., flash memory, EEPROM). The digitallycontrolled stimulator executing the TCI event-sequencing time line alsocommunicates with a CPU (not shown) based data storage and analysisapparatus to direct binning or storing of responses and to triggerarchival data storage, analysis and display paradigms.

[0117] In addition to an indication of which stimulator is active andwhether adequate current delivery is achieved, there is preferably alsoan additional indicator annunciating detection of an adequate targetimpedance, thereby providing a rough quality check of the stimulus probeand the entire stimulator circuit. This type of diagnostic would be bestapplied to the flush tip stimulus probe designs (as in U.S. Pat. No.4,892,105), where the impedance is typically related to thecross-sectional area of the conductor contact surface.

[0118] The controller software used in monitoring the stimulus probeimpedance detection circuit 40 (or current flow detection circuit 36)includes an algorithm for identifying a pattern of changing impedance(or current flow change) caused by double or triple taps of probe 50against patient tissue. When double or triple tap patterns are detected,signals are sent to the circuitry in the CPU digital interface 32 fortriggering predetermined manipulations. These command signals arepreferably rendered “context sensitive” by their temporal occurrence inrelation to the TCI-Time line.

[0119] Returning now to FIG. 4 the intraoperative neurophysiologicalmonitoring system 100 includes digitally controlled stimulator currentflow detection circuit CD-1, 106 and a second, downstream current sourceCS-2, 104, and is well suited to performing the method of the presentinvention with current flow detection only. Current source 108 comprisesthe main source of current to provide nerve stimulation; althoughseparate current sources for each stimulus output may be employed, asingle source is shown.

[0120] Current Source #2, 104 provides continuous, sub-threshold currentthrough cathode of stimulus output probe 102 for detection oftissue-contact. As above, the switches or current gates CG-1, CG-2 andCG-3 are actuated or driven by tissue contact detection. The presentdesign uses current-flow detection as the means to detecttissue-contact. The switches (“current-gates”) are kept in theopen-position until tissue-contact is detected at one of the cathodeterminals at the output which produces a signal from the correspondingimpedance-detection circuit to close the electronic switch for the probe(e.g., 102) in contact with tissue, and opens the others, as above, theswitches are configured so that only one switch can be closed at a time.Each stimulus output also has a stimulus-controller SC-1 that effectsthe intensity, duration and temporal patterns of deliveredstimulus-pulses. The controller is, in turn, controlled by adigital-interface DI-1. Current-flow will be measured for each stimulusoutput by a current-flow detection circuit (e.g. CD-1). Secondcurrent-source 104 injects a continuous, sub-threshold current beyondthe current-gate CG-1, which is used for the detection oftissue-contact. During delivery of “stimulus-current” the output of theCS-2 circuit is used to drive the digital-readout of current-flow forthe corresponding stimulus output (e.g., probe 102).

[0121] The output of Current Flow Detector (CFD) CD-1, relating tomeasured “stimulus-current” flow, is compared against the“user-intended” level by a “comparator” circuit 112. If the value fallswithin predetermined limits (90-95%), the comparator circuit 112 putsout an “enable” signal used to trigger an “adequate-current”speech-sample or tone, and preferably also incorporated as an “enable”signal for the TCI-Time line.

[0122] Digital interface DI-1, 114 stores various stimulus-paradigmswhich are initiated in a pre-programmed fashion (TCI-Time line) bydetection of tissue-contact of the primary stimulus probe 102. Digitalinterface 114 also directs data capture, analysis, display and storagein a pre-programmed fashion per the TCI-Time line. Interface 114consists of two components, one of which is located in the main unit,the other of which is located in a PCI-bus slot in the computer. Thedigital interface 114 controls a stimulus controller SC-1, 116 whichshapes the current provided from current source 108 into stimuli ofpre-programmed intensity, duration, and temporal pattern. The digitalinterface DI-1, 114 also controls functions relating to dataacquisition, analysis, display, and storage through a connection with aCPU (not shown).

[0123] For stimulus outputs #2, #3 or both (shown only through the SCsegments), the digital-interface may be configured to inputmeasured-values of stimulus-circuit impedance and make pre-programmedadjustments of stimulus intensity, based upon measured-impedance values.It is anticipated that stimulus output #1 (shown in its entirety) willbe used with a flush-tip stimulus probe 102 for which such anapplication is unnecessary.

[0124]FIG. 5. is a flow diagram illustrating the current flow detectionalgorithm for detection of tissue contact, for use with the monitoringsystem stimulator current flow detection circuit of FIG. 4, inaccordance with the present invention. Once probe 102 is brought intocontact with the tissue of a patient, a small probe current from secondcurrent source 104 is sensed and current gate or switch CG-1 is closedto provide stimulus current. Substantially simultaneously, the TCI-timeline algorithm is initiated and, optionally, a light emitting diode(LED) is illuminated, thus providing an indication of “appropriate”tissue contact impedance. Stimulus current flows through probe 102 andthe nerve tissue and the stimulus current flow is detected in currentdetection circuit CD-1, 106; if the detected current is of theappropriate magnitude, an LED is illuminated, thus providing anindication of “appropriate” current flow. Once the surgeon lifts theprobe and interrupts tissue contact, current flow stops and the lack ofcurrent flow is detected with current detection circuit 106, whereuponswitch CG-1 is open circuited in response and the TCI-Time linealgorithm is reset.

[0125]FIG. 6 is a top view of an artifact detection electrode 130 foruse during intraoperative neurophysiological monitoring to provide areliable means of detecting electromagnetic and current artifacts,occurring in the physical-proximity of multiple active recordingelectrodes. Signal output from artifact-detection electrode 130 is usedin a simple logic paradigm for the purposes of distinguishingelectromagnetic (EM) and current artifacts from biophysiologicalresponses, and is useful to detect when general anesthesia is becominginadequate or light. Probe 130 is well suited for detection andidentification of artifacts as an aid to interpretation, and can beplaced in different groups of muscles to obtain different measurements.For the purposes of this description, “intelligent” refers to electrodesites Involving important “monitored” muscles, supplied or enervated bya particular nerve of Interest. Non-intelligent refers to otherelectrode sites within or outside of muscles, not supplied by the nerveof Interest. Current artifacts and electromagnetic field noise may bestbe detected by electrode 130 when inserted proximate to the recordingfield, but not in the (intelligent) muscles supplied by the nerve beingmonitored. Electrical events, simultaneously recorded in both“intelligent” electrodes (placed in muscles supplied by the nerve beingmonitored) and a “non-intelligent” artifact detection electrode, may beunambiguously interpreted as electrical artifacts. If the artifactdetection electrode is placed in a nearby (non-intelligent) muscle notsupplied by the nerve being monitored, it may also serve to detect lightanesthesia. If repetitive EMG activity is simultaneously observed inmonitored muscles and other muscles, it may be interpreted that thepatient is beginning to wake up from anesthesia. The anesthesiologistmay use this information to maintain adequate levels of anesthesiathroughout the procedure. The operating surgeon may also be reassuredthat the observed nerve irritability is not related to surgicalmanipulations. This artifact detection strategy is abetted by theconstruction of artifact-detection electrode 130 which is a modificationof the electrode design of U.S. Pat. No. 5,161,533 (as discussed above).The modification provides a greater impedance imbalance between the twoelectrode leads 132, 134, thereby reliably enhancing the antenna-likequalities of the probe and the susceptibility for detecting current andelectromagnetic artifacts occurring in the immediate proximity ofmultiple electrodes placed in muscles supplied by the nerve of interest.

[0126] Artifact detection electrode 130 has an active-portion that issimilar to the paired, bipolar Teflon coated needle electrodes, butdiffers in that the area of un-insulated needle 136, 138 is dimensionedand/or made of a suitable material to provide a reliably detectableimpedance imbalance.

[0127] Preferably, wire leads 132, 134 are also modified such that thelead length is approximately 6 inches longer than standard length. Theextra 6-inch portion is looped over the recording field to create,effectively, an antenna 139 over the recording field. The looped portionis treated to enhance its antenna-like properties. Optionally, incombination with or instead of using differing uninsulated areas ofneedle insertion portion, a resistor 140 is placed in series with one ofthe two electrode leads, thereby creating a readily detected impedanceimbalance, the value of which may be selected (or, with a potentiometer,adjusted) to be within a range of, preferably, zero to approximately50,000 ohms. Resistor 140 is preferably located on the wire lead or loop139, or it may be incorporated into an associated electrical connectorhousing or connector body (e.g., 144). A relative disadvantage of usinga single standard recording electrode for detection of electromagneticfield and current artifacts is that the single electrode may notadequately represent the electromagnetic field for multiple activerecording electrodes. The loop design, needle to insulation symmetry,fixed resistor value and relative location are the physical factorsdetermining the “antenna like” properties of the electrode design; thevarious features are preferably “tuned” to obtain the optimum electrodecharacteristics. The electrode must be spatially selective enough toavoid pick up of “intelligent” signal, but must have adequate antennalike qualities to provide EM-field and current artifact detection torepresent the entire recording field.

[0128] The uninsulated portion of the electrode needles 136, 138 of theartifact detection electrode 130 is placed in a proximate,“non-intelligent” muscle, not enervated or supplied by the nerve beingmonitored. The looped portion 139 of the electrode lead is placed overthe recording field of the intelligent electrodes and held in place,preferably with tape.

[0129] The artifact-detection electrode output is detected and analgorithm incorporating a simple artifact-recognition strategy, basedupon response distribution, is employed. The signal output of theartifact detection electrode is amplified along with that of standard“intelligent” electrodes. Brief supra-threshold signal episodes (approx.<1 sec.), detected in intelligent electrodes, trigger a logic-circuit toevaluate for simultaneous signal in the artifact-detection electrode.Simultaneous detection of supra-threshold signal in theartifact-detection electrode renders an interpretation of “artifact.” Ifno simultaneous signal is detected in the artifact-detection electrode,the episode is interpreted as EMG in the algorithm, since it is highlyunlikely that two different nerves are simultaneously (mechanically orelectrically) stimulated.

[0130] For repetitive EMG activity lasting from several seconds toseveral minutes, detection of activity among “intelligent” electrodesindicates irritability in the nerve of interest, which may be due tosurgical manipulations, whereas simultaneous detection of activity inintelligent and non-intelligent electrodes are interpreted as inadequateor “light” anesthesia, because surgically-evoked repetitive-EMG activityis otherwise unlikely to occur simultaneously in two distinct musclegroups.

[0131] An example of such an artifact detection strategy is the use of amasseter muscle electrode during facial nerve monitoring. The massetermuscle is in the proximate electromagnetic field of the facial muscles,but is not enervated by the facial nerve. Brief electromagnetic andcurrent events that are simultaneously detected in facial and massetermuscles are readily interpreted as artifacts. Further, when repetitiveactivity is detected in masseter and facial electrodes, it suggests thatthe anesthesia is getting light.

[0132] The intraoperative neurophysiological monitoring system of thepresent invention includes a controller circuit and software algorithmsto identify and categorize artifacts based upon the observeddistribution among “intelligent” and “non-intelligent” electrode sites.In one embodiment, a logic circuit receives output from thresholddetection circuits related to both “intelligent” and “non intelligent”electrode sites. When a supra threshold signal is detected in one of the“intelligent” electrode sites, the circuit becomes activated to make adetermination regarding whether the signal detected was likely to havebeen artifact or true EMG. At the time of supra threshold signaldetection in one (or more) of the “intelligent” channels, the output ofthe “non intelligent” channel threshold detection circuit is checked forsimultaneous activation (using, e.g., a logic AND gate). If there was nosupra threshold activity in the “non intelligent” channel, the logiccircuit produces an output signal indicating that the observed activitywas “true EMG”. If simultaneous supra threshold activity was detected inboth the “intelligent” and “non-intelligent” channels, the logic circuitproduces an output signal indicating that the observed activity waslikely to have been a non-EMG artifact.

[0133] The accuracy of the present artifact-detection strategy isdependent upon the strength of the recorded signal. Weak signals thatonly appear in a single channel may not distribute among intelligent andnon-intelligent electrodes as predictably as when multiple electrodesare activated.

[0134] If more than one “intelligent” channel (and electrode) isutilized, the logic circuit is preferably configured to allow a userselected requirement to produce an output signal indicating the identityof a supra threshold signal as “true EMG” or “artifact” only when two ormore “intelligent” channels are simultaneously activated by suprathreshold signals. This will increase the accuracy of the logic circuitdeterminations, reduce the frequency at which the circuit gives falsepositive feedback, and indicate a response of greater magnitude andprobable significance.

[0135] The novel artifact-detection electrode and logical strategy fordistinguishing electrical artifacts and EMG signals of the presentinvention works with simple threshold detection involving analog voltagemeasurement, but simple threshold detection has significant limitationsfor this application. One disadvantage is that repetitive EMG activity,caused by persistent nerve irritability, impairs the ability to detectmore important episodes of non-repetitive EMG activity. Repetitiveactivity swamps the threshold detection circuit and causes repetitivedetection of supra threshold events.

[0136] In the present embodiment, threshold detection is improvedthrough the use of digital signal processing (DSP), whereby all recordedelectrical activity is digitized and evaluated for mathematicalproperties. A preferred measurement for EMG activity is rectified rootmean square (rRMS), which gives a greater dynamic range for EMG activitymagnitude, as detected by standard electrodes (e.g., as in U.S. Pat. No.5,161,533, discussed above). The greater dynamic range capabilityimproves the ability to distinguish responses, based upon the magnitudeof signal power. For example, while electrical artifacts and EMGresponses show considerable overlap, the peak signal power of anon-repetitive (localizing) EMG activity is usually significantly higherthan for a repetitive (non-localizing) EMG activity. The digitallyprocessed rRMS data stream for each recording channel is continuouslyanalyzed by software for peak and average power within a variable time(probe) window. The width of the probe window (or dwell) over whichpower is analyzed may be varied in width (duration) up to one second,which may be “tuned” to give desired fractionating tendencies.

[0137] Another aspect of the present invention is an artifact detectionmethod for use during intraoperative neurophysiological monitoring andpreferably in conjunction with electrode 130, which isspecifically-designed and used for detection of artifacts. Additionally,the present invention involves a circuit that is specifically designedto identify artifacts based upon the observed distribution among“intelligent” and “non-intelligent” electrode sites.

[0138] A simple logic-circuit receives output from threshold detectioncircuits related to both “intelligent” and “non-intelligent” electrodesites. When a supra-threshold (i.e., over threshold) signal is detectedin one of the “intelligent” electrode sites, the circuit becomesactivated to make a determination regarding whether the signal detectedwas likely to have been artifact or true EMG. At the time ofsupra-threshold signal detection in one (or more) of the “intelligent”channels, the output of the “non-intelligent” channel thresholddetection circuit is checked for simultaneous activation. If there wasno supra threshold activity in the “non-intelligent” channel, thelogical circuit will produce an output signal, indicating that theobserved activity was likely to be true-EMG. If simultaneoussupra-threshold activity was observed in the “non-intelligent” channelwas detected, the logical circuit will produce an output signal,indicating that the observed activity was likely to have been artifact.

[0139] Preferably, more than one “intelligent” channel is utilized andso the logical circuit is configured to only become activated (i.e., tomake a logical determination) when two or more “intelligent” channelsare simultaneously activated, thereby increasing the accuracy of thelogical-circuit determinations, reducing the frequency at which thecircuit gives feedback, and indicating to the surgeon when there hasbeen a more significant response.

[0140] If DSP analysis of “intelligent” signals is used for the purposeof artifact identification, the output of that separate determinationmay be fed into the present logical circuit. Depending upon whether ornot the DSP-related determination agrees with the presentdistribution-related determination, the logical circuit output mightgenerate an appropriate signal to indicate “highly-probable,”“probable,” “possible,” or “inconclusive,” depending upon thedifferential weighting given to the respective methods of determination.

[0141] Turning now to FIGS. 7, 8 and 9, another aspect of the presentinvention relates to a versatile, precise and ergonomic method ofcontrol for multiple data-management procedures associated withintraoperative neurophysiological monitoring. The method (discussedabove in conjunction with the TCI time line algorithm) involves digitalcontrol of a preprogrammed array of electrical stimuli and a coordinatedseries of data acquisition, analysis, display and storage algorithmsinitiated through the detection of the temporal pattern of electricalstimulus probe use and is particularly advantageous in the field ofintraoperative electromyographic (EMG) monitoring in association withperiods of electrical stimulus probe use. Certain aspects of the controlsystem may be linked to supra-threshold detection of EMG or artifactactivity. Moreover, the method and algorithm may be adapted to otherfields in which a probe is used for data acquisition and wheredata-management operations can be linked to monitored aspects of itsuse.

[0142]FIG. 7 is a graphical representation of a pre-programmed set ofelectrical stimulus pulses of varying intensities used in theintraoperative monitoring of responses to stimulus pulses, incombination with a flow diagram illustrating the steps in the TCI-timeline algorithm and the context sensitive interrupt commands forcontrolling whether the TCI time line algorithm is aborted or completed.FIG. 8 is a block diagram of an intraoperative neurophysiologicalmonitoring system 200 with a TCI-Time line algorithm controlledimpedance and current flow detection circuit, and FIG. 9 is a blockdiagram of an alternative, simpler embodiment including anintraoperative neurophysiological monitoring system 300 with a TCI-Timeline algorithm controlled current flow detection trigger circuit, inaccordance with the present invention.

[0143] Referring now to the upper portion of FIG. 7, showing a graphicalrepresentation of a pre-programmed set of electrical stimulus pulses ofvarying intensities used in the intraoperative monitoring of responsesto stimulus pulses, the vertical axis is graduated in milliamps (mA) ofstimulus current applied through a stimulus probe and the horizontalaxis overhead is a time scale in seconds. A preprogrammed pattern orparadigm of stimulus pulses, as illustrated, preferably includes a firstpair 150 of stimulus pulses spaced at less than 100 mS apart and havingequal amplitudes of approximately 0.10 mA, these are called pairedpulses 150 and are followed at a spacing of approximately 100 mS by asingle pulse 152 having an equal amplitude, 0.10 mA. Preferably, thepattern next includes another set of paired pulses 150, followed inalternate succession by another single pulse 152.

[0144] The steps of the algorithm are illustrated in the center portionof FIG. 7, in which later steps are below the step before. TheTCI-algorithm has two parallel or simultaneous processes, as will bedescribed in greater detail below.

[0145] Returning to FIG. 8, intraoperative neurophysiological monitoringsystem 200 includes current source 208 which generates stimulus currentand is connected to the stimulus controller 210 which controls theintensity, duration and temporal patterns of delivered stimulus pulses.Controller 210 is, in turn, responsive to and controlled by adigital-interface (DI-1) 204. Current-flow is measured for each stimulusoutput by a current-flow detection circuit 212, the output of thiscircuit will be used to drive the digital-readout of current-flow forthe corresponding stimulus output. The output of the current flowdetection circuit 212, relating to measured current-flow, is comparedagainst the user selected level by a comparator circuit 214, and if thevalue falls within predetermined limits (90-95%), the comparator circuit214 optionally puts out an “enable” signal, to be used to trigger an“adequate-current” speech sample or tone; it may also be incorporated asan “enable” signal for the TCI-Time line. Digital-interface 204 storesvarious stimulus-paradigms, which are initiated in a pre-programmedfashion (TCI-Time line) by detection of tissue-contact detection. Thedigital interface also directs data capture, analysis, display andstorage in a preprogrammed fashion per the TCI-Time line. The interfaceconsists of two components, one of which is located in the main unit,the other of which is located in a PCI-bus slot in the computer. Foroptional

[0146] For output #2, #3 or both, the digital-interface may beconfigured to input measured-values of stimulus-circuit impedance andmake pre-programmed adjustments of stimulus intensity, based uponimpedance values. It is anticipated that stimulus output #1 will be usedwith a flush-tip stimulus probe for which such an application is notnecessary. Impedance detection circuit ID-1 220 provides an indicationof tissue-contact and to measure nominal stimulus-circuit impedance.Detection of tissue-contact can be used to initiate the TCI-Time lineand measurement of impedance can be used to provide a “quality-check” ofthe stimulus-circuit integrity and provide a means of adjusting stimulusintensity to the level of current-shunting. For impedance measurement,the impedance-detection circuit provides a small, sub-threshold signalthat is detected to establish continuity. Patient connections forimpedance detection circuit 220 is electrically or optically isolated byisolation circuit 222. A comparator INDEPENDENT CLAIM-1 224 receivesoutput from impedance detection circuit 220 and computes scalarrepresentations or values of stimulus circuit impedance and provides anoutput digital interface 204 to drive the various data-handlingoperations and preprogrammed stimulus intensity adjustments. Digitalinterface 204 connects the stimulator with a CPU 206, so that dataacquisition, analysis, display and storage can be coordinated. Digitalinterface 204 and CPU 206 execute spreadsheeting of data and drive agraphic display 208 (e.g., a CRT or LCD). Digital interface 204 ispreferably configured to direct the capture of digitally-sampled audioand video data corresponding to signal data. CPU 206 is preferablyprogrammed to store files for later retrieval and “off-line” analysis.

[0147] As shown in FIG. 9, intraoperative neurophysiological monitoringsystem 300 includes current source 308 which generates stimulus currentand is connected to the stimulus controller 310 which controls theintensity, duration and temporal patterns of delivered stimulus pulses.Controller 310 is, in turn, responsive to and controlled by adigital-interface (DI-1) 304. Current-Source #2, CS-2, 309 injects asmall, continuous, sub-threshold current as a probe signal to providemeans of tissue-contact detection. Current-flow is measured for eachstimulus output by a current-flow detection circuit 312, the output ofthis circuit will be used to drive the digital-readout of current-flowfor the corresponding stimulus output. The output of the current flowdetection circuit 312, relating to measured current-flow, is comparedagainst the user selected level by a comparator circuit 314, and if thevalue falls within predetermined limits (90-95%), the comparator circuit314 optionally puts out an “enable” signal, to be used to trigger an“adequate-current” speech sample or tone; it may also be incorporated asan “enable” signal for the TCI-Time line.

[0148] As noted above, quantitative measurements of nerve function inintraoperative monitoring are relatively cumbersome and requireinvolvement of technical personnel to change stimulator settings andvarious recording parameters in order to acquire, analyze, display andstore data. The applicant has noted that there are not many types ofquantitative measurements regarding nerve function assessment, however,and that threshold and peak amplitude measurements are the most widelyused. The applicant has also discovered that paired stimuli pulses 150are particularly effective when assessing nerve fatigue. Operatingsurgeons usually have specific preferences regarding the type ofquantitative data to be collected and analyzed during the course of agiven surgical procedure, so there is little need for “on-the-fly”flexibility in the operating room (OR) when performing quantitative datacollection.

[0149] Quantitative data on nerve function is mainly acquired throughthe use of an electrical stimulus probe (e.g., 202 or 302, as shown inFIGS. 8 and 9), provoking electromyographic responses for quantitativeanalysis.

[0150] The inventor has observed that surgeons use the stimulus probe(e.g., 202) differently for locating and “Mapping” than for quantitativeanalysis of the functional status of nerves of interest. Temporalaspects of stimulus probe use can be monitored by the tissue contactdetection capability within the digital stimulator as describedpreviously. A signal is generated in the stimulator that relates to theperiod of continuous contact of the stimulator probe with patienttissue. The signal continues as long as continuous tissue contact ismaintained and is delivered to a system controller, which is able toinitiate multiple predetermined sequential and parallel operationswithin the nerve integrity monitor, as shown in the central flow chartportion of FIG. 7. These operations relate to delivery of preprogrammedstimulus sequences and to the acquisition, analysis, display andarchival storage of EMG data. Whether the predetermined operations areinitiated or completed depends upon the duration of continuous tissuecontact. For example, as shown in the flowchart portion at the bottom ofFIG. 7, if the duration continuous tissue contact is less than apreselected period of approximately one or two seconds, the controllerwill maintain the operational status of the nerve Integrity monitor inthe “search” mode. However, if the duration of continuous tissue contactexceeds the preselected time period, the stimulator or controller mayalert the surgeon with an indicator tone and controller willautomatically change the operational status of the nerve integritymonitor to a quantitative assessment mode and provide a preprogrammedsequence of quantitative assessment stimulus pulses 160, as shown in theupper portion of FIG. 7. A monitoring indicator tone may also bedesigned or configured to signify whether adequate current and/orstimulator circuit impedance has been achieved, as an indication ofquality assurance, as discussed above. From the time of tissue contactdetection, a digital clock is initiated, controlling a preprogrammedsequence of events through a controller interface DI-1 (as shown inFIGS. 8 and 9). For the purposes of this description, the period ofcontinuous tissue contact of the stimulus probe is termed the “dwell” or“dwell time”, and the series of preselected operational changes provokedby the “dwell” is termed, the “Tissue Contact Initiated Event SequencingTime line” or“TCI-Time line” and is illustrated, in part, in the middleand loser flow charts included in FIG. 7. The control method to bedescribed is designed for use with the main stimulus probe (e.g., probes202, 302 connected to stimulus output #1 in a multi-stimulus outputsystem, as described above) and may be used to control all functions ofthe nerve integrity monitor in a preselected fashion, by execution of analgorithm stored in memory. The described methodology need not belimited to medical applications, in that the use of any probe, where itsperiod of dwell can be measured, may be similarly configured to controlmultiple functions. The following description involves the preferredembodiment, although many possible sequence strategies are availablethrough the TCI-Time line:

[0151] Through the associated controller and controller interface (e.g.the Digital interface DI-1 204, of FIG. 8 and 304, of FIG. 9), the onsetof dwell causes the artifact-detection circuit to be suspended(“defeated”) throughout its duration and a preset pattern of stimuluspulses, the intensity of which is determined by front panel controls,will be delivered through the stimulator probe for locating and“mapping” the physical contour of the nerve of Interest. After apreselected dwell time of approximately one second (as shown in theupper part of FIG. 7), front panel control of stimulus parameters isdefeated, the pattern of stimuli is changed from single pulses 152 toalternating paired pulses 150 with single pulses 152, the intensity ofwhich is somewhat greater (supra maximal), and the provoked EMGresponses are digitized and individually captured into stable buffers.If the dwell is interrupted before a dwell of 2 seconds, the TCI-Timeline is inactivated or aborted, the artifact-detection circuit Isenabled, the stable buffers are cleared of captured signal and pulsedstimuli are no longer delivered through the stimulus probe. After a 2second preselected period of dwell, the controller and associatedinterface initiate a signal processing sequence, where the capturedresponses in stable buffers are analyzed by averaging the single andpaired responses separately and computing the difference between thepaired and single response by digital subtraction. The magnitude of thesingle and digitally subtracted responses are computed and compared. Ascalar value relating to a ratio of the magnitudes of the digitallysubtracted response and the single response is stored in a spreadsheetagainst the absolute or lapsed time (of the operation) and is displayedby CRT output automatically or upon an input “request” by the operatingsurgeon. The stable buffers used in these computations are automaticallycleared at completion. The above computational operations occur inparallel to the following:

[0152] After a 2 second preselected period of dwell, the controller andinterface defeat front panel control of stimulus parameters and alterthe stimulus delivery pattern to a series of single pulses of varyingintensity 160. The controller and interface direct the provoked EMGresponses to be captured individually into stable buffers. If the dwellis interrupted prior to completion of the stimulus sequence, theTCI-Time line is discontinued, the sequence of stimulator pulses isdiscontinued, the stable buffers are cleared of captured signal, theartifact-detection functions are enabled and stimulus parameters arereverted to front panel controls. However, interruption of the dwellafter 2 seconds does not Interfere with the completion of the paralleloperations described above regarding the mathematical treatment of EMGactivity-provoked by single and paired stimulus pulses.

[0153] If the dwell is continued (after 2 seconds), then until thestimulus sequence is completed, the stimulator or TCI-Time linecontroller delivers a second indicator tone and the controller andinterface initiate a series of operations to generate a scalar value ofresponse threshold. Each individually captured EMG response is analyzedfor power content (peak or average), the scalar value of which is storedin a spreadsheet in conjunction with the stimulus intensity used toprovoke it. The spreadsheet data relating to all stimulus intensitiesand corresponding responses is used to compute (or estimate) thestimulus intensity in milliamps (mA) at which half-maximal responsemagnitude (power) occurred. This scalar value (in mA) is then defined asthe “response threshold” and is applied to a spreadsheet againstabsolute or lapsed time of the surgical procedure. The scalar value or agraphical plot of threshold versus operative time may be displayedautomatically by CRT screen or displayed upon request by input suppliedby the operating surgeon. These computational operations are carried outin parallel with progress of the dwell and may reach completionconsiderably after the dwell has been interrupted.

[0154] As described, the “TCI-Time line” Is a multidimensional controlalgorithm or device utilizing information spanning both time and space.The continuous tissue contact dwell serves to initiate various series ofoperations through the TCI-Time line controller and interface. Theseoperations may include simple or complex stimulus delivery paradigms,and corresponding data acquisition, analysis, display and archivalstorage procedures. The stimulation sequences and data handlingalgorithms proceed along different time lines, as per pre-programmed,parallel (processing) software algorithms. As long as the dwellcontinues, these operations proceed to completion in sequence.Alternatively, interruption of the dwell aborts all subsequentinitiation of events along the dwell, but may allow some of thepreviously initiated events to reach completion as described above. TheTCI-Time line controller directs operational events in differentlocations within the nerve integrity monitoring device. Production ofstimulus pulses occurs in the stimulator portion of the monitor, whiledata acquisition, analysis, display and storage may occur in differentlocations, such as on the memory of a PCI card, CPU RAM memory or a harddrive. Thus the present TCI-Time line control system must account formultiple time dimensions and multiple locations within the monitoringdevice.

[0155] Detection of tissue contact is preferably achieved by continuousstimulator circuit impedance measurement or continuous measurement ofcurrent flow with use of a separate sub-threshold current delivereddownstream from actual pulsed stimuli to the patient. Either of thesemethods will allow the detection of the temporal pattern caused bytapping the stimulator probe two or three times onto patient tissue(away from Important structures) as a means of providing additionalinput to the controller through the tissue contact detection circuit. A“double” or “triple” tap of the stimulus probe may be preselected foraltering the normal operation of the controller, such as initiating adisplay of previously stored data as a “time trend.” That is, a “doubletap” command may provoke the controller to display a time trend of ameasured parameter, such as response threshold. The scalar value ofstimulus intensity (mA), where the response threshold is achieved, isplotted against time (duration of the operation) to give the surgeon aclearer impression of how the nerve of interest has responded throughoutthe surgical procedure.

[0156] Optionally, the control capabilities of the TCI-Time line areused for analyzing and storing data derived from detection of suprathreshold events. Supra threshold events may transferred from stablebuffers, described previously with regard to “additional DSP” analysisof supra threshold events, and converted to file format for archivalstorage. The file of the digitized signal, its scalar DSP values (e.g.,peak and average rRMS), and its channel number (or identity) may bearchived (as in a spreadsheet) against the absolute or lapsed(operative) time of its appearance for later (off-line) retrieval. Suchcapabilities improve the ability to “tune” DSP parameters for greateraccuracy in detecting appropriate events for analysis, for alerting theoperating surgeon and for distinguishing artifacts from true EMG.

[0157] Preferably, audio and video capture devices are integrated intothe system to perform audio and video data capture functions. Anindependent method of distinguishing artifact and EMG supra thresholdevents is to interpret events in the context of the surgical procedure.If the supra threshold event occurred exactly at the time of a surgicalmanipulation, it may be interpreted as a mechanically stimulated (hencenon-repetitive) EMG event. Alternatively, if the event appears to occurindependently of surgical manipulations it is interpreted as eitherartifact or non-localizing (repetitive) EMG. Relatively brief (3-5seconds) periods of digitized audio signal of the sound delivered to thesurgeon through the loudspeaker in the nerve integrity monitor anddigitized video of the surgical procedure, from a (microscope or handheld) camera monitoring the surgical field, is adequate to interpret the“context” of a supra threshold event. Audio and video signal may bedigitized and held in FIFO “scroll” buffers within the nerve integritymonitor. For investigational purposes, the logical circuits used fordetection of supra threshold events may send a signal to the TCI-Timeline controller when certain preselected supra threshold events aredetected; the signal provokes the TCI-Time line controller to cause thecapture of digitized audio and video for an interval starting 2-4seconds before and ending one second after the onset of the suprathreshold event. The captured audio and video can then be converted tofile form (*.avi, *.mpg or equivalent) and archived along with thesignal data mentioned above. Such capability tremendously facilitatesevaluation (validation) of various methods of event (artifact and EMGresponse type) detection for accuracy and effectiveness.

[0158] With the present control system, temporal aspects of stimulusprobe use can be made to control an entire quantitative analysisparadigm in a pre-programmed, preset manner, based upon the needs of theuser. This will involve a mix of sequential and parallel operations andsmooth operation is dependent upon a seamless digital CPU interface(e.g., 204 or 304) for control of data acquisition, analysis anddisplay, preferably in a Windows® based software system. The algorithmsteps or command sequences and interrupt interpretations are stored onnon volatile memory, such as EEPROM or “flash memory,” providing fastonline operation in a controller which is readily reprogrammed ormodified off-line by CPU-interface. At present, the prevailing standarddigital interface is the Peripheral Components Interface (PCI); it is tobe understood that future developments may provide equivalents to thePCI standard. Accordingly, the following discussion is a description ofbut one exemplary embodiment which happens to include a PCI circuitcard.

[0159] The enhanced or “complete” neurophysiological monitoring system200 (as shown in FIG. 8) consists of the basic monitoring unit, aprocessor 204 including a CPU 206 (e.g., an Intel Pentium® brandmicroprocessor) and a Peripheral Components Interface (PCI) circuitcard. CPU 206 interfaces with the basic monitoring unit through the PCIfor both off-line and on-line operations. Digitized signals from thebasic monitoring unit are continually delivered (e.g., via an opticaltransmission link) to the PCI card, which continually routes them totemporary scroll buffers. When triggered by the tissue contact initiated(TCI) Time line or by detection of evoked EMG responses, recorded signalevents are “captured,” along with time, data channel identification andother relevant information. The captured signals are held in a stablebuffer for DSP manipulations (e.g., Fast Fourier Transform (FFT)frequency conversion) and for conversion to a selected file format. Ascroll buffer is a first-in-first-out (FIFO) image buffer storing themost recent waveform segment; the stored segment has a selected duration(e.g., approx. 2-10 seconds). A stable buffer (or bin) is also an imagebuffer but only holds discrete supra-threshold waveforms or events, andso effectively ignores the waveform trace between events; the stablebuffer holds waveforms of selected durations or epoch lengths (e.g.,approximately 1 second).

[0160] The PCI interface includes the scroll buffers and the stablebuffers containing captured signal data for quantitative facial nervesignal assessment. Associated DSP circuitry is located on a PCI circuitboard. There must be a relatively generous number of stable buffers (orbins) available to separately capture one or more given EMG events onmultiple channels and to capture individual responses relating tostimuli of differing parameters (intensities). Additional buffer spacesor bins must also be available for digital subtraction functions, wherea “third” bin stores the computed difference between two others forfurther quantitative analysis. The total number of bins must be adequateto handle a variety of analysis algorithms.

[0161] There must also be consideration for how signals occurringsimultaneously or nearly simultaneously will be processed. Theindividual bin size must be adequate to store a large number of samples,thereby providing adequate waveform fidelity and the sample rate ortime-base must be high enough to capture signals with the requiredaccuracy.

[0162] The processor includes a motherboard having a CPU for acquisitionof scalar data from the DSP circuits on the PCI-card and for datapresentation functions such as spreadsheeting and graphing (e.g., usingLotus® or Excel® spreadsheet programs) and for controlling the display.The processor is preferably also configured to create, tag and bundledata files from the temporary stable buffers on the PCI card. Image datafiles are preferably bundled with corresponding “captured” audio andvideo files (from separate video and sound cards) and then transferredinto permanent storage in appropriate locations on the processor harddrive for later review. Off-line, saved data is readily re-loaded intotemporary stable buffers to permit the surgeon to review or re-analyzedata to observe the effectiveness of artifact recognition and nervefunction assessment.

[0163] Thus, the system delegates DSP functions to various componentsfor rapid performance of mathematical operations and display of data.Complex stimulation paradigms in the form of software algorithms areinitiated by a digitally controlled stimulator, based upon temporalaspects of tissue contact by the main stimulus probe. The digitalstimulator (or the controller executing the TCI-Time line algorithm)sends simultaneous signals through the PCI-interface to direct data tothe appropriate buffers (or bins) for on-line analysis. Additionalsignals, either from the basic monitoring unit or internally generatedon the PCI by pre-programmed algorithms, initiate pre-set data-displayand data storage algorithms. Six to twelve different stimuli and acorresponding number of storage buffers may be employed for thresholddetection. Alternating paired and single pulses will require at leastthree bins. One each for binning responses evoked by paired and singlepulses, and a third for holding computed digital subtraction data.Optionally, within the two bins for single and paired responses or bycombining the results of separate bins, repetitious responses may beused to compute a signal “average” for single and paired responses. Therespective averages may be used to compute the digital subtraction datafor the “third” bin.

[0164] Complete control over on-line operations of the intraoperativeneurophysiological monitor of the present invention can be achievedthrough the use of the TCI-Time line and is preferably set up off-lineusing keyboard and mouse input devices through a standard personalcomputer operating system such as Microsoft Windows® software.

[0165] In the preferred embodiment all changes made by off-line inputprocedures are transferred to the Main unit of the nerve integritymonitor and “burned in” to non-volatile (EEPROM or flash) memory. As aresult, the information transferred will be protected from spuriousvoltage spikes and accidental unplugging. This Is distinct from priorart methodology, where off-line changes are stored in volatile memory,which may be susceptible to spurious voltage spikes and accidentalunplugging of equipment.

[0166] Additional on-line flexibility is afforded through use of simpleinput devices which are convenient and easy to use, but not ascomprehensive as the keyboard and mouse combination; in one embodiment,the stimulus probe is used as a pointing device for inputs to thecontroller, as will be described in greater detail, below.

[0167] Yet another aspect of the present invention is an adaptablethreshold level setting method for use during intraoperativeneurophysiological monitoring and is also intended to enhance detectionof brief episodes of EMG activity, provoked by mechanical or electricalstimuli. The method improves the performance and accuracy of the abovedescribed artifact-detection strategy, based upon response distributionamong “intelligent” and “non-intelligent” electrodes.

[0168] Threshold-detection is based on measured signal power (such asroot-mean-square) is monitored for each channel. As signal powerincreases, the threshold is automatically elevated in order to avoidthreshold detection of background EMG activity. One embodiment of thismethod is to sample signal-power at intervals, and to hold thedeterminations in temporary memory, such as in a digital scroll method.In order for a supra-threshold, signal event to be detected, one or moreconsecutive signal power determinations would have to be greater, by apreset difference level, than the signal power sampled one second beforethem. An alternative is to require that one or more consecutivesignal-power determinations be greater than the power levels one secondbefore and one-second after, thereby limiting threshold-detection tojust brief responses.

[0169] In an alternative and preferred embodiment the thresholding couldbe performed in conjunction with determination of the likelihood that anobserved event is non-repetitive EMG activity instead of the morenoise-like repetitive EMG activity. The method includes defining“probes” or sampling windows of time which stored waveform traces arepassed through. FIG. 10 is a set of related waveform traces, plotted asa function of time, illustrating first and second probe sampling windows350, 352 each of a selected duration and temporally spaced at a selectedinter-probe interval 354. FIG. 11 is a waveform trace, plotted withvoltage as a function of time, illustrating a non-repetitive EMGactivity 360, and FIG. 12 is a waveform trace, plotted with voltage as afunction of time, illustrating a repetitive EMG activity 362. FIG. 13 isa set of related waveform traces, plotted with voltage as a function oftime, illustrating a non-repetitive EMG activity 360 superposed on (oroccurring and sensed simultaneously with) a repetitive EMG activity 362.FIG. 14 is a set of related waveform traces, plotted with voltage as afunction of time, illustrating a sensed non-repetitive EMG signaltemporally situated between first and second probe sampling windows ofselected durations, and the Rectified RMS power derived from thedifference detection algorithm for first and second probe samplingwindow durations; and FIG. 15 is a set of related waveform traces,plotted with voltage as a function of time, illustrating a sensedrepetitive EMG signal of a duration including first and second probesampling windows of selected durations, and the Rectified RMS powerderived from the difference detection algorithm, for first and secondprobe sampling window durations. FIG. 16 is a set of related waveformtraces, plotted with voltage as a function of time, illustrating asensed non-repetitive EMG signal temporally situated between first andsecond probe sampling windows and superposed on a sensed repetitive EMGsignal of a duration including the first and second probe samplingwindows, and the Rectified RMS power derived from the differencedetection algorithm, for a selected probe sampling window duration. Asdiscussed above, the intraoperative neurophysiological monitoring systemalso includes an enhanced method and algorithm for detecting orthresholding non-repetitive EMG events or activity (such as the shortduration pulses 360 indicative of EMG activity) as distinguished from arepetitive EMG activity waveform 362, even when the non-repetitive EMGevents are sensed simultaneously with the repetitive EMG events, inwhich case the waveforms are superposed upon one another as shown inFIG. 13. The enhanced threshold detection algorithm includes the stepsof buffering or storing a continuous series of samples of the sensed EMGwaveforms from one or more sensing electrodes (e.g., 130); the bufferedwaveform is processed by running the stored waveform samples seriallythrough spaced probe first-in-first-out (fifo) sampling windows ofselected duration and having a selected temporal spacing therebetween;in the preferred embodiment, the probe sampling windows (e.g., 350, 352)have a duration in the range of 0.25 seconds to 0.5 seconds and thebeginning of the first probe sampling window is temporally spaced (withan inter-probe interval 354) at one second from the beginning of thesecond probe sampling window. The algorithm passes the stored waveformsamples serially through first probe sampling window and then throughthe second probe sampling window. As the stored waveform samples (e.g.,360) pass through each probe sampling window (350, 352), a scalar valuecorresponding to the rectified RMS (rRMS) power of the waveform isgenerated. As best seen in FIGS. 14, 15 and 16, the algorithmcontinuously computes a threshold value by subtracting the instantaneousvalue of the second probe sampling window rRMS power from the firstprobe sampling window rRMS power; the continuously generated results ofthis computation are readily plotted as a threshold value waveform 370.Since the algorithm passes the stored waveform samples serially throughthe first probe sampling window and then through the second probesampling window, a non-repetitive EMG activity will produce thethreshold value waveform 370 (of FIG. 14) having a first, positive goingpulse 372 having a width approximating the duration of thenon-repetitive EMG activity 360 (corresponding to the first probesampling window rRMS power) and then a second negative going pulse 374having the same width (corresponding to the subtracted second probesampling window rRMS power).

[0170] Alternatively, as best seen in FIG. 15, a repetitive EMG activity362 having a duration longer than the selected (1.0 second) spacingbetween the probe sampling windows produces a threshold value waveform380 having only one positive going pulse having a width approximatingthe duration of the interval beginning at the start of the first probesampling window 350 and ending at the start of the second probe samplingwindow 352 (in the present example, a duration of one second). For astored waveform having a non-repetitive EMG activity superposed on arepetitive EMG activity, as shown in FIG. 16, the algorithm will producea threshold value waveform 384 having a first one second long pulseincluding a second positive going pulse having a width approximating theduration of the non-repetitive EMG activity (corresponding to the firstprobe sampling window rRMS power) and then a second negative going pulsehaving the same width as the non-repetitive EMG activity (correspondingto the subtracted second probe sampling window rRMS power).

[0171] Whenever the enhanced threshold detection algorithm produces athreshold value waveform including a first positive going pulse followedby a second negative going pulse, there is an indication that a brief(e.g., <1.0 sec) response has occurred which may be either localizingnon-repetitive EMG or artifact. Detection of such an event provokes theartifact-detection circuitry to evaluate its spatial distribution among“intelligent” and “non-intelligent” electrodes and (optionally)additional DSP algorithms in order to determine its status as anartifact or (localizing) EMG event. The surgeon is then prompted with anappropriate audible and (optionally) visual annunciation.

[0172] Insofar as monitoring instrument use is concerned, additionalon-line flexibility is afforded through use of simple input deviceswhich are convenient and easy to use, but not as comprehensive as thekeyboard and mouse combination; in one embodiment, the stimulus probe(e.g., 202) is used as a pointing device for inputs to the controller.During surgery, or when “on-line”, an electrical stimulus probe ispreferably employed as a convenient controller input device and theTCI-Time line algorithm controls most on-line system operations,including which data are displayed to the operating surgeon on the CRTscreen display, however, the surgeon may periodically want to seeadditional information, such as a display of a measured parametergraphed as a function of time, over course of the procedure. Thestimulus probe provides a convenient and simple input device forinitiating such requests, since the surgeon is likely already holdingthe probe, and so need not put the probe down to use a keyboard, or thelike. The TCI-Time line algorithm is triggered upon detection of tissuecontact by the electrical stimulus probe. Tissue contact detectionincludes probe signal current flow or impedance-change detection.

[0173] In addition to providing an indication of presence or absence oftissue contact, the tissue contact detection apparatus is configured torecognize specific signatures, such as a “double tap” or “triple tap” ofthe-stimulus probe against non-sensitive patient tissue within thesurgical field. The detection of these predetermined signatures can beused to provide additional online input to the TCI-Time line controller.When such a pattern is detected, a separate signal is sent to theTCI-Time line controller for initiation of context sensitive,predetermined commands, a sequence analogous to a “double click” of astandard mouse when pointing to an icon in a Windows® compatibleprogram. The identity of these commands are changeable, depending uponthe monitoring context of the request; context is provided by theTCI-Time line algorithm. If the “double-click” occurs before thecompletion of a TCI-Time line controlled operation, the request isinterpreted differently than for a double-click occurring aftercompletion.

[0174] The tapping pattern can differ among different users, in orderfor the tapping pattern of a given user is recognized, a setup algorithmincludes an adjustment method allowing the user to input his or herindividual tapping pattern. Recognition of tapping patterns may beperformed by “default” recognition settings within the tissue contactdetection circuitry. However, because the temporal aspects of tappingmay vary significantly among individual surgeons, the preferred systemallows an individual surgeon's tapping signature to be captured forlater recognition. It is preferred that this is performed early in thesurgical procedure, before critical stages. For this procedure, a frontpanel or foot pedal switch is depressed, immediately after which thesurgeon performs a “double tap” or “triple tap” signature. The patternof impedance change or current flow change detected by the tissuecontact detection circuitry is stored and used as a template forrecognition of similar “signature” patterns at a later time.

[0175] Also, when the double- or triple-tap input command is used, asound sample or audible annunciation is preferably activated to indicatethat the intended command has been successfully communicated. The soundsample might can be any form of effective audible feedback to the user(e.g., a sound of a standard mouse double-click or triple-click).

[0176] A double tap is defined as a first probe contact having a first,short duration (e.g., less than one second) followed by an interval inwhich the probe is lifted and not in contact with anything and havingsecond, short duration (e.g., also less than a second) and followed by athird probe contact having a third, short duration (e.g., less than onesecond). A computer executable algorithm (preferably part of theTCI-Time line algorithm) for detecting and responding to the double tapsequence is readily prepared and responds to sensed current or impedancechanges indicating the one, two or three taps has occurred, and then, inresponse, triggers execution of a desired monitoring or data handling ordisplay oriented command.

[0177] After completion of a TCI-Time line controlled, pre-programmedstimulus sequence with corresponding quantitative data display, thealgorithm preferably includes program steps for detecting a stimulusprobe double-tap and, in response, displaying all similar measurementsobtained from the beginning of the procedure (e.g., traced as a waveformshowing voltage as a function of time), wherein a time-trend ofstimulation threshold can be observed to detect a significant injury inprogress. Similarly, after supra-threshold detection of an EMG response,the algorithm may include “if then” condition detection program stepswherein detection of a “double tap” is the input causing a display ofthe IDSP data for that response or for a display of a DSP-derivedparameter, such as root mean square (RMS) power, as a function of time.Such a trend may show a loss of signal power over the course of theprocedure and may indicate a fatigue trend in the nerve underobservation in response to ongoing mechanical manipulations.

[0178] A simple input device used in conjunction with the TCI-Time linealgorithm alternatively includes two or three button operated switchesaccessed from a cylindrical handle. The two button configuration used ina manner similar to setting of a watch; one button selects options froma menu displayed on the nerve integrity monitor and the other button isused to choose a user preference or selection from the menu of options.Alternatively, a three-button input device provides more flexibilitywith forward and backward movement through a menu or series of menus,since the buttons could be used to scroll up, scroll down or selectoption, respectively. The simple input device is readily kept sterile onthe field and its simplicity allows rapid data or control input and easeof use. Such a device does not require the use of the stimulating probe.

[0179] The above described simple devices for on-line use provide inputthrough the monitoring system controller digital interface, rather thanthrough a serial port of the host computer. Off-line operations,controlled by keyboard and mouse, preferably operate through mouse andkeyboard ports on the controller CPU.

[0180] Turning now to another aspect of the present invention, a squelchcontrol method is provided for use during multi-channel intraoperativeneurophysiological monitoring for the purposes of enhancing thesurgeon's ability to hear brief localizing (non-repetitive)electromyographic responses during periods of significant backgroundactivity. The squelch control method is based upon the method fordetecting repetitive EMG activity made possible by the enhancedthreshold detection strategy described above. Data from all (andexclusively) “intelligent” EMG channels is digitized and monitored bythe enhanced threshold detection circuit, employing two probe windows asdescribed, with an inter-probe interval of approximately one second. ByDSP, the average rRMS is continuously computed for both windows and thescala value is referenced against electrical silence. With the two probewindow strategy, if only one window is active at a time, the duration ofa supra threshold event must be less than the inter-probe interval. Ifboth windows are active simultaneously, the duration is equal to orgreater than the inter-probe interval. Since the vast majority ofnon-repetitive activity is less than one second in duration, aninterprobe interval of one second is able to effectively distinguishrepetitive and non-repetitive responses. Repetitive responses aredetected when both probe windows are simultaneously active.

[0181] In the “automatic” embodiment of the present invention, thescalar values of average rRMS derived from the two probe windows arecontinuously scanned by a software comparator constructed innon-volatile memory. The comparator is configured to compare ongoingaverage rRMS values against a user preselected threshold value. If thethreshold value is exceeded in both probe windows, a signal is generatedwhich activates a muting switch to eliminate that particular channelfrom the audio (loudspeaker) signal to the operating surgeon. If otherchannels reach supra threshold levels of continuous repetitive EMGactivity, more channels may be muted, except the last (quietest)channel. That is, no matter how much repetitive activity, at least one“intelligent” channel is preserved for continuous audio display of EMGsignals to the operating surgeon. When the average rRMS values of bothwindows decrease below threshold levels, the muting switch isautomatically disabled.

[0182] In an alternative embodiment, the muting function can be enabledmanually. Some surgeons may prefer to decide on a “case by case” basis,when to begin muting offending EMG channels. When bothered by persistentrepetitive EMG activity, the surgeon may request that a nurse ortechnician depress a momentary push-button switch, conveniently locatedon the front panel of the nerve integrity monitor. With activation ofthe push button switch, all (except the quietest) channels with suprathreshold levels of repetitive EMG activity are muted from the audiosignal to the surgeon. As with the previous “automatic” embodiment, oncethe activity has quieted to sub-threshold levels, the audio output isautomatically re-enabled. It is preferred that the surgeon be given theoption of automatic and manual operation by a simple front panel controlselections.

[0183] Having described preferred embodiments of a new and improvedmethod, it is believed that other modifications, variations and changeswill be suggested to those skilled in the art in view of the teachingsset forth herein. It is therefore to be understood that all suchvariations, modifications and changes are believed to fall within thescope of the present invention as defined by the appended claims.

What is claimed is:
 1. A method for intraoperative neurophysiologicalmonitoring with at least one electrical stimulus probe as anintraoperative aid in defining the course of a nerve structure bymonitoring electromyographic activity within the nerve structure,comprising: (a) contacting the nerve structure with the stimulus probe;(b) detecting a stimulus probe impedance change resulting from saidstimulus probe nerve contact; (c) triggering a sequence ofpre-programmed intraoperative neurophysiological monitoring algorithmsteps in response to the detection of step (b).
 2. The method forintraoperative neurophysiological monitoring of claim 1, wherein step(c) comprises closing a circuit between a current source and thestimulus probe to provide stimulus current to the nerve structure. 3.The method for intraoperative neurophysiological monitoring of claim 1,wherein step (c) comprises: (c)(1) generating a visible or audibleannunciation of appropriate nerve tissue contact impedance.
 4. Themethod for intraoperative neurophysiological monitoring of claim 3,wherein said step (c)(1) comprises: (c)(2) generating an annunciationsignal to illuminate a Light Emitting Diode.
 5. The method forintraoperative neurophysiological monitoring of claim 1, wherein step(c) comprises: (c)(1) initiating generation of a pre-programmed sequenceof stimulus pulses; and (c)(2) storing the measured responses collectedfrom electrodes connected to the enervated muscle structures.
 6. Themethod for intraoperative neurophysiological monitoring of claim 5,further comprising: (c)(3) analyzing the stored, measured responsescollected from electrodes connected to the enervated muscle structures,to determine the average response amplitude.
 7. The method forintraoperative neurophysiological monitoring of claim 5, furthercomprising: (c)(3) analyzing the stored, measured responses collectedfrom electrodes connected to the enervated muscle structures, todetermine the peak-to-peak response amplitude.
 8. The method forintraoperative neurophysiological monitoring of claim 5, furthercomprising: (c)(3) analyzing the stored, measured responses collectedfrom electrodes connected to the enervated muscle structures, todetermine the response threshold.
 9. The method for intraoperativeneurophysiological monitoring of claim 5, further comprising: (c)(3)analyzing the stored, measured responses collected from electrodesconnected to the enervated muscle structures, to determine the responseamplitude as a function of stimulus intensity.
 10. A method forintraoperative neurophysiological monitoring, comprising: (a) placing afirst electrode in a muscle enervated by a selected nerve; (b) placing asecond electrode in a muscle not enervated by the selected nerve; (c)stimulating the selected nerve; (d) monitoring the effect of saidstimulation step as observed from the first electrode and simultaneouslymonitoring the effect of said stimulation step as observed from thesecond electrode; and (e) actuating an audible or visible alarm if theeffect of the stimulation is observed on the first electrode but not onthe second electrode.
 11. The method for intraoperativeneurophysiological monitoring of claim 10, further comprising: (b1 )placing a third electrode in a muscle not enervated by the selectednerve; and (d1) monitoring the effect of said stimulation step asobserved from the first electrode and simultaneously monitoring theeffect of said stimulation step as observed from the third electrode.12. A method for detecting and analyzing a neurophysiological signal inthe body, comprising the steps of: (a) defining a first probe samplingwindow of time having a first selected duration; (b) defining a secondprobe sampling window of time having a second selected duration andbeing delayed with respect to said first probe sampling window of timeby a selected inter-probe interval of time; (c) contacting a nervestructure in the body; (d) sensing a continuous and time varyingelectromyographic waveform from the nerve structure; (e) storing saidnerve structure electromyographic waveform in memory; (f) rectifyingsaid nerve structure electromyographic waveform; and (g) generating acontinuous threshold waveform by processing the rectified nervestructure electromyographic waveform through said first probe samplingwindow and through said second probe sampling window and subtracting theinstantaneous value of the waveform power in said second probe windowfrom the instantaneous value of the waveform power in said first probewindow.
 13. The method for detecting and analyzing a neurophysiologicalsignal of claim 12, further comprising the steps of: (h) determiningwhether said continuous threshold waveform includes a first pulse havinga first polarity followed by a second pulse having a second polarity byan interval substantially equal to said selected inter-probe interval;and if so, (i) generating an annunciation indicating that an artifacthas been detected.
 14. The method for detecting and analyzing aneurophysiological signal of claim 13, further comprising the steps of:(j) evaluating the distribution among intelligent and non-intelligentelectrodes in response to said annunciation that an artifact has beendetected.
 15. The method for detecting and analyzing aneurophysiological signal of claim 12, further comprising the steps of:(h) determining whether said continuous threshold waveform includes afirst pulse having a pulse width substantially equal to said selectedinter-probe interval followed by an interval having no pulse and beingsubstantially equal to said selected inter-probe interval; and if so,(i) generating an indication that no artifact has been detected.
 16. Themethod for detecting and analyzing a neurophysiological signal of claim12, wherein said first probe sampling window first selected duration isequal to said second probe sampling window second selected duration. 17.The method for detecting and analyzing a neurophysiological signal ofclaim 16, wherein said first probe sampling window first selectedduration is in the range of 0.25 seconds to 0.5 seconds.
 18. The methodfor detecting and analyzing a neurophysiological signal of claim 16,where said selected inter-probe interval is approximately one second.19. A method for controlling a neurophysiological monitoring instrumentconnected to a stimulus probe and one or more electrodes for monitoringelectromyographic activity in nerve and muscle structures in the body,comprising: (a) connecting a first circuit to the stimulus probe forsensing an electrical parameter that changes in response to touchingtissue structures in the body, said first circuit being adapted togenerate a stimulus probe sensed signal pulse; (b) connecting acontroller connected to said first circuit for receiving said stimulusprobe sensed signal, said controller being adapted to execute analgorithm including a plurality of instrument control commands and aplurality of selected patterns of stimulus probe sensed signal pulses;(c) placing a stimulus probe in proximity to tissue structures in thebody; (d) sensing said electrical parameter changing in response totouching tissue structures in the body; (e) generating a pattern ofstimulus probe sensed signal pulses similar to one of said selectedpatterns of stimulus probe sensed signal pulses; and (f) generating aninstrument control command in response to detecting said pattern ofstimulus probe sensed signal pulses.
 20. The method for controlling aneurophysiological monitoring instrument connected to a stimulus probeof claim 19, wherein said instrument control command changes theinstrument display mode.
 21. The method for controlling aneurophysiological monitoring instrument connected to a stimulus probeof claim 19, wherein said instrument control command changes theinstrument stimulus signal amplitude.
 22. The method for controlling aneurophysiological monitoring instrument connected to a stimulus probeof claim 19, wherein said instrument control command changes theinstrument stimulus signal frequency.
 23. The method for controlling aneurophysiological monitoring instrument connected to a stimulus probeof claim 19, wherein said instrument control command begins generationof a predefined pattern of stimulus signal pulses.
 24. The method forcontrolling a neurophysiological monitoring instrument connected to astimulus probe of claim 19, wherein said instrument control commandchanges the instrument audible annunciation mode, enabling generation ofaudible tones.
 25. The method for controlling a neurophysiologicalmonitoring instrument connected to a stimulus probe of claim 19, whereinsaid instrument control command changes the instrument audibleannunciation mode, disabling generation of audible tones.
 26. The methodfor controlling a neurophysiological monitoring instrument connected toa stimulus probe of claim 19, wherein said instrument control commandcauses a command represented by an icon on the instrument display screento be executed.
 27. An artifact detection electrode for sensingneurophysiological signal artifacts in tissue structures, in the body,comprising: a first electrode needle having an insulated proximalportion and a sharp, uninsulated distal portion; a second electrodeneedle having an insulated proximal portion and a sharp, uninsulateddistal portion; a first elongate conductor having a proximal end, anintermediate portion and a distal end; said first elongate conductorbeing electrically connected to said first electrode needle proximalportion at said first conductor distal end; said second elongateconductor being electrically connected to said second electrode needleproximal portion at said second conductor distal end; said firstelongate conductor having a circuit element connected in seriestherewith; and said first conductor intermediate segment and said secondconductor intermediate portion being configured in a loop to define arecording field signal receiving antenna.
 28. The artifact detectionelectrode for sensing neurophysiological signal artifacts of claim 27,said first elongate conductor having said circuit element connected inseries in said first elongate conductor intermediate segment.
 29. Theartifact detection electrode for sensing neurophysiological signalartifacts of claim 27, said first elongate conductor having said circuitelement connected in series and disposed in a connector body affixed tosaid proximal end of said first elongate conductor.
 30. The artifactdetection electrode for sensing neurophysiological signal artifacts ofclaim 27, said resistive circuit element being a fixed value resistor.31. The artifact detection electrode for sensing neurophysiologicalsignal artifacts of claim 27, said resistive circuit element being avariable resistance potentiometer.
 32. The artifact detection electrodefor sensing neurophysiological signal artifacts of claim 31, saidvariable resistance potentiometer having an adjustment range of fromzero ohms to approximately fifty thousand ohms.